<chapter name="Les Houches Accord"> 
 
<h2>Les Houches Accord</h2> 
 
The Les Houches Accord (LHA) for user processes <ref>Boo01</ref> is the 
standard way to input parton-level information from a 
matrix-elements-based generator into PYTHIA. The conventions for 
which information should be stored has been defined in a Fortran context, 
as two commonblocks. Here a C++ equivalent is defined, as a single class. 
The most common application is to read input from a Les Houches Event File 
(LHEF) <ref>Alw06</ref>, but it is also possible to have a runtime 
interface to another program. This page will discuss runtime interface. For 
LHEF handling, please refer to 
<aloc href="LHEF">Les Houches Event Files</aloc>. 
 
<p/> 
The <code>LHAup</code> class is a base class, containing reading and 
printout functions, plus two pure virtual functions, one to set 
initialization information and one to set information on each new event. 
Derived classes have to provide these two virtual functions to do 
the actual work. The existing derived classes are for reading information 
from a Les Houches Event File, from the respective Fortran 
commonblocks, or from PYTHIA 8 itself. 
 
<p/> 
You are free to write your own derived classes, using the rules and 
methods to be described below. Normally, pointers to objects of such 
derived classes should be handed in with the 
<code><aloc href="ProgramFlow">Pythia::setLHAupPtr( LHAup*)</aloc></code> 
method. However, with the LHEF format a filename can replace the 
pointer, see further below. 
 
<p/> 
Let us now describe the methods at your disposal to do the job. 
 
<method name="LHAup::LHAup( int strategy = 3)"> 
the base class constructor takes the choice of mixing/weighting 
strategy as optional input argument, and calls <code>setStrategy</code>, 
see below. It also reserves some space for processes and particles. 
</method> 
 
<method name="virtual LHAup::~LHAup()"> 
the destructor does not need to do anything. 
</method> 
 
<method name="void LHAup::setPtr(Info* infoPtr)"> 
this method only sets the pointer that allows some information 
to be accessed, and is automatically called by 
<code>Pythia::init()</code>. 
</method> 
 
<h3>Initialization</h3> 
 
The <code>LHAup</code> class stores information equivalent to the 
<code>/HEPRUP/</code> commonblock, as required to initialize the event 
generation chain. The main difference is that the vector container 
now allows a flexible number of subprocesses to be defined. For the 
rest, names have been modified, since the 6-character-limit does not 
apply, and variables have been regrouped for clarity, but nothing 
fundamental is changed. 
 
<method name="virtual bool LHAup::setInit()"> 
this pure virtual method has to be implemented in the derived class, 
to set relevant information when called. It should return false if it 
fails to set the info. In the no-beams extension this method need not 
do anything, since by default strategy 3 is chosen and the rest is set 
vanishing, but the method must exist. 
</method> 
 
<p/> 
Inside <code>setInit()</code>, such information can be set by the following 
methods: 
<method name="void LHAup::setBeamA( int identity, double energy, 
int pdfGroup = 0, int pdfSet = 0)"> 
</method> 
<methodmore name="void LHAup::setBeamB( int identity, double energy, 
int pdfGroup = 0, int pdfSet = 0)"> 
sets the properties of the first and second incoming beam, respectively 
(cf. the Fortran <code>IDBMUP(1), EBMUP(i), PDFGUP(i), PDFSUP(i)</code>, 
with <code>i</code> 1 or 2). These numbers can be used to tell which PDF 
sets were used when the hard process was generated, while the normal 
<aloc href="PDFSelection">PDF Selection</aloc> is used for the further 
event generation in PYTHIA. 
</methodmore> 
 
<method name="void LHAup::setStrategy( int strategy)"> 
sets the event weighting and cross section strategy. The default, 
provided in the class constructor, is 3, which is the natural value 
e.g. for an LHEF. 
<argument name="strategy"> 
chosen strategy (cf. <code>IDWTUP</code>; see <ref>Sjo06</ref> 
section 9.9.1 for extensive comments). 
<argoption value="1"> events come with non-negative weight, given in units 
of pb, with an average that converges towards the cross section of the 
process. PYTHIA is in charge of the event mixing, i.e. for each new 
try decides which process should be generated, and then decides whether 
is should be kept, based on a comparison with <code>xMax</code>. 
Accepted events therefore have unit weight.</argoption> 
<argoption value="-1"> as option 1, except that cross sections can now be 
negative and events after unweighting have weight +-1. You can use 
<code><aloc href="EventInformation">Info::weight()</aloc></code> 
to find the weight of the current event. A correct event mixing requires 
that a process that can take both signs should be split in two, one limited 
to positive or zero and the other to negative or zero values, with 
<code>xMax</code> chosen appropriately for the two.</argoption> 
<argoption value="2"> events come with non-negative weight, in unspecified 
units, but such that <code>xMax</code> can be used to unweight the events 
to unit weight. Again PYTHIA is in charge of the event mixing. 
The total cross section of a process is stored in 
<code>xSec</code>.</argoption> 
<argoption value="-2"> as option 2, except that cross sections can now be 
negative and events after unweighting have weight +-1. As for option -1 
processes with indeterminate sign should be split in two.</argoption> 
<argoption value="3"> events come with unit weight, and are thus accepted 
as is. The total cross section of the process is stored in 
<code>xSec</code>.</argoption> 
<argoption value="-3"> as option 3, except that events now come with weight 
+-1. Unlike options -1 and -2 processes with indeterminate sign need not be 
split in two, unless you intend to mix with internal PYTHIA processes 
(see below).</argoption> 
<argoption value="4"> events come with non-negative weight, given in units 
of pb, with an average that converges towards the cross section of the 
process, like for option 1. No attempt is made to unweight the events, 
however, but all are generated in full, and retain their original weight. 
For consistency with normal PYTHIA units, the weight stored in 
<code>Info::weight()</code> has been converted to mb, however. 
</argoption> 
<argoption value="-4"> as option 4, except that events now can come 
either with positive or negative weights.</argoption> 
<note>Note 1</note>: if several processes have already been mixed and 
stored in a common event file, either LHEF or some private format, it 
would be problematical to read back events in a different order. Since it 
is then not feasible to let PYTHIA pick the next process type, strategies 
+-1 and +-2 would not work. Instead strategy 3 would be the recommended 
choice, or -3 if negative-weight events are required. 
<note>Note 2</note>: it is possible to switch on internally implemented 
processes and have PYTHIA mix these with LHA ones according to their relative 
cross sections for strategies +-1, +-2 and 3. It does not work for strategy 
-3 unless the positive and negative sectors of the cross sections are in 
separate subprocesses (as must always be the case for -1 and -2), since 
otherwise the overall mixture of PYTHIA and LHA processes will be off. 
Mixing is not possible for strategies +-4, since the weighting procedure 
is not specified by the standard. (For instance, the intention may be to 
have events biased towards larger <ei>pT</ei> values in some particular 
functional form.) 
</argument> 
</method> 
 
<method name="void LHAup::addProcess( int idProcess, double xSec, 
double xErr, double xMax)"> 
sets info on an allowed process (cf. <code>LPRUP, XSECUP, XERRUP, 
XMAXUP</code>). 
Each new call will append one more entry to the list of processes. 
The choice of strategy determines which quantities are mandatory: 
<code>xSec</code> for strategies +-2 and +-3, 
<code>xErr</code> never, and 
<code>xMax</code> for strategies +-1 and +-2. 
</method> 
 
<note>Note</note>: PYTHIA does not make active use of the (optional) 
<code>xErr</code> values, but calculates a statistical cross section 
error based on the spread of event-to-event weights. This should work 
fine for strategy options +-1, but not for the others. Specifically, 
for options +-2 and +-3 the weight spread may well vanish, and anyway 
is likely to be an underestimate of the true error. If the author of the 
LHA input information does provide error information you may use that - 
this information is displayed at initialization. If not, then a relative 
error decreasing like <ei>1/sqrt(n_acc)</ei>, where <ei>n_acc</ei> 
is the number of accepted events, should offer a reasonable estimate. 
 
<method name="void LHAup::setXSec( int i, double xSec)"> 
update the <code>xSec</code> value of the <code>i</code>'th process 
added with <code>addProcess</code> method (i.e. <code>i</code> runs 
from 0 through <code>sizeProc() - 1</code>, see below). 
</method> 
 
<method name="void LHAup::setXErr( int i, double xErr)"> 
update the <code>xErr</code> value of the <code>i</code>'th process 
added with <code>addProcess</code> method. 
</method> 
 
<method name="void LHAup::setXMax( int i, double xMax)"> 
update the <code>xMax</code> value of the <code>i</code>'th process 
added with <code>addProcess</code> method. 
</method> 
 
<method name="void LHAup::setInfoHeader(string &key, string &val)"> 
set the header <code>key</code> to have value <code>val</code>. 
This is a wrapper function to the 
<aloc href="EventInformation">Info::setHeader</aloc> function that 
should be used in any classes derived from LHAup. 
</method> 
 
<p/> 
Information is handed back by the following methods 
(that normally you would not need to touch): 
<method name="int LHAup::idBeamA()"> 
</method> 
<methodmore name="int LHAup::idBeamB()"> 
</methodmore> 
<methodmore name="double LHAup::eBeamA()"> 
</methodmore> 
<methodmore name="double LHAup::eBeamB()"> 
</methodmore> 
<methodmore name="int LHAup::pdfGroupBeamA()"> 
</methodmore> 
<methodmore name="int LHAup::pdfGroupBeamB()"> 
</methodmore> 
<methodmore name="int LHAup::pdfSetBeamA()"> 
</methodmore> 
<methodmore name="int LHAup::pdfSetBeamB()"> 
for the beam properties. 
</methodmore> 
<method name="int LHAup::strategy()"> 
for the strategy choice. 
</method> 
<method name="int LHAup::sizeProc()"> 
for the number of subprocesses. 
</method> 
<method name="int LHAup::idProcess(int i)"> 
</method> 
<methodmore name="double LHAup::xSec(int i)"> 
</methodmore> 
<methodmore name="double LHAup::xErr(int i)"> 
</methodmore> 
<methodmore name="double LHAup::xMax(int i)"> 
for process <code>i</code> in the range <code>0 &lt;= i &lt; 
sizeProc()</code>. 
</methodmore> 
<method name="double LHAup::xSecSum()"/> 
<methodmore name="double LHAup::xErrSum()"> 
the sum of the cross sections and errors (the latter added quadratically). 
Note that cross section errors are only meaningful for strategies +-3. 
</methodmore> 
 
<method name="void LHAup::listInit()"> 
prints the above initialization information. This method is 
automatically called from <code>Pythia::init()</code>, 
so would normally not need to be called directly by the user. 
</method> 
 
<p/> 
 
 
<h3>Event input</h3> 
 
The <code>LHAup</code> class also stores information equivalent to the 
<code>/HEPEUP/</code> commonblock, as required to hand in the next 
parton-level configuration for complete event generation. The main 
difference is that the vector container now allows a flexible number 
of partons to be defined. For the rest, names have been modified, 
since the 6-character-limit does not apply, and variables have been 
regrouped for clarity, but nothing fundamental is changed. 
 
<p/> 
The LHA standard is based on Fortran arrays beginning with 
index 1, and mother information is defined accordingly. In order to 
be compatible with this convention, the zeroth line of the C++ particle 
array is kept empty, so that index 1 also here corresponds to the first 
particle. One small incompatibility is that the <code>sizePart()</code> 
method returns the full size of the particle array, including the 
empty zeroth line, and thus is one larger than the true number of 
particles (<code>NUP</code>). 
 
<method name="virtual bool LHAup::setEvent(int idProcess = 0)"> 
this pure virtual method has to be implemented in the derived class, 
to set relevant information when called. For strategy options +-1 
and +-2 the input <code>idProcess</code> value specifies which process 
that should be generated, while <code>idProcess</code> is irrelevant 
for strategies +-3 and +-4. The method should return 
false if it fails to set the info, i.e. normally that the supply of 
events in a file is exhausted. If so, no event is generated, and 
<code>Pythia::next()</code> returns false. You can then interrogate 
<code><aloc href="EventInformation">Info::atEndOfFile()</aloc></code> 
to confirm that indeed the failure is caused in this method, and decide 
to break out of the event generation loop. 
</method> 
 
<p/> 
Inside a normal <code>setEvent(...)</code> call, information can be set 
by the following methods: 
<method name="void LHAup::setProcess( int idProcess, double weight, 
double scale, double alphaQED, double alphaQCD)"> 
tells which kind of process occurred, with what weight, at what scale, 
and which <ei>alpha_EM</ei> and <ei>alpha_strong</ei> were used 
(cf. <code>IDPRUP, XWTGUP, SCALUP, AQEDUP, AQCDUP</code>). This method 
also resets the size of the particle list, and adds the empty zeroth 
line, so it has to be called before the <code>addParticle</code> method below. 
</method> 
<method name="void LHAup::addParticle( int id, int status, int mother1, 
int mother2, int colourTag1, int colourTag2, double p_x, double p_y, 
double p_z, double e, double m, double tau, double spin, double scale)"> 
gives the properties of the next particle handed in (cf. <code>IDUP, ISTUP, 
MOTHUP(1,..), MOTHUP(2,..), ICOLUP(1,..), ICOLUP(2,..),  PUP(J,..), 
VTIMUP, SPINUP</code>; while <code>scale</code> is a new optional property, 
see further below) . 
</method> 
 
<p/> 
Information is handed back by the following methods: 
<method name="int LHAup::idProcess()"> 
process number. 
</method> 
 
<method name="double LHAup::weight()">. 
Note that the weight stored in <code>Info::weight()</code> as a rule 
is not the same as the above <code>weight()</code>: the method here gives 
the value before unweighting while the one in <code>info</code> gives 
the one after unweighting and thus normally is 1 or -1. Only with strategy 
options +-3 and +-4 would the value in <code>info</code> be the same as 
here, except for a conversion from pb to mb for +-4. 
</method> 
 
<method name="double LHAup::scale()"> 
</method> 
<methodmore name="double LHAup::alphaQED()"> 
</methodmore> 
<methodmore name="double LHAup::alphaQCD()"> 
scale and couplings at that scale. 
</methodmore> 
 
<method name="int LHAup::sizePart()"> 
the size of the particle array, which is one larger than the number 
of particles in the event, since the zeroth entry is kept empty 
(see above). 
</method> 
 
<method name="int LHAup::id(int i)"> 
</method> 
<methodmore name="int LHAup::status(int i)"> 
</methodmore> 
<methodmore name="int LHAup::mother1(int i)"> 
</methodmore> 
<methodmore name="int LHAup::mother2(int i)"> 
</methodmore> 
<methodmore name="int LHAup::col1(int i)"> 
</methodmore> 
<methodmore name="int LHAup::col2(int i)"> 
</methodmore> 
<methodmore name="double LHAup::px(int i)"> 
</methodmore> 
<methodmore name="double LHAup::py(int i)"> 
</methodmore> 
<methodmore name="double LHAup::pz(int i)"> 
</methodmore> 
<methodmore name="double LHAup::e(int i)"> 
</methodmore> 
<methodmore name="double LHAup::m(int i)"> 
</methodmore> 
<methodmore name="double LHAup::tau(int i)"> 
</methodmore> 
<methodmore name="double LHAup::spin(int i)"> 
</methodmore> 
<methodmore name="double LHAup::scale(int i)"> 
for particle <code>i</code> in the range 
<code>0 &lt;= i &lt; sizePart()</code>. (But again note that 
<code>i = 0</code> is an empty line, so the true range begins at 1.) 
</methodmore> 
 
<p/> 
From the information in the event record it is possible to set 
the flavour and <ei>x</ei> values of the initiators 
<method name="void LHAup::setIdX(int id1, int id2, double x1, double x2)"> 
</method> 
 
<p/> 
This information is returned by the methods 
<method name="int LHAup::id1()"> 
</method> 
<methodmore name="int LHAup::id2()"> 
</methodmore> 
<methodmore name="double LHAup::x1()"> 
</methodmore> 
<methodmore name="double LHAup::x2()"> 
the flavour and <ei>x</ei> values of the two initiators. 
</methodmore> 
 
<p/> 
In the LHEF description <ref>Alw06</ref> an extension to 
include information on the parton densities of the colliding partons 
is suggested. This optional further information can be set by 
<method name="void LHAup::setPdf( int id1pdf, int id2pdf, double x1pdf, 
double x2pdf, double scalePDF, double pdf1, double pdf2, bool pdfIsSet)"> 
which gives the flavours , the <ei>x</ei> and the <ei>Q</ei> scale 
(in GeV) at which the parton densities <ei>x*f_i(x, Q)</ei> have been 
evaluated. The last argument is normally <code>true</code>. 
</method> 
 
<p/> 
This information is returned by the methods 
<method name="bool LHAup::pdfIsSet()"> 
</method> 
<methodmore name="int LHAup::id1pdf()"> 
</methodmore> 
<methodmore name="int LHAup::id2pdf()"> 
</methodmore> 
<methodmore name="double LHAup::x1pdf()"> 
</methodmore> 
<methodmore name="double LHAup::x2pdf()"> 
</methodmore> 
<methodmore name="double LHAup::scalePDF()"> 
</methodmore> 
<methodmore name="double LHAup::pdf1()"> 
</methodmore> 
<methodmore name="double LHAup::pdf2()"> 
where the first one tells whether this optional information has been set 
for the current event. (<code>setPdf(...)</code> must be called after the 
<code>setProcess(...)</code> call of the event for this to work.) 
Note that the flavour and <ei>x</ei> values usually but not always 
agree with those obtained by the same methods without <code>pdf</code> 
in their names, see explanation in the 
<aloc href="EventInformation">Event Information</aloc> description. 
</methodmore> 
 
<p/> 
The maximum scale for parton-shower evolution of a Les Houches event is 
regulated by the 
<code><aloc href="TimelikeShowers">TimeShower:pTmaxMatch</aloc></code> 
and 
<code><aloc href="SpacelikeShowers">SpaceShower:pTmaxMatch</aloc></code> 
modes. If you want to guarantee that the input <code>scale</code> value 
is respected, as is often the case in matching/merging procedures, you 
should set both of these modes to 1. That only affects the hard process, 
while resonance decays are still processed using the resonance mass to 
set the upper limit. However, the optional 
<code><aloc href="BeamParameters">Beams:strictLHEFscale = on</aloc></code> 
setting restricts also resonance-decay emissions to be below the input 
<code>scale</code> value. 
 
<p/> 
As a further non-standard feature, it is also possible to read in the 
separate scale values of all final particles. Such scale values could be used 
e.g. to restrict the maximum scale for shower evolutions for each parton 
separately. This reading will only be applied if the <code> 
Beams:setProductionScaleFromLHEF</code> switch is true (see <code> 
<aloc href="BeamParameters">Beam Parameters</aloc></code> for details). 
This information is returned by the method 
<code>double LHAup::scale(int i)</code>. When no such information 
has been read from the LHEF, the scale defaults to -1. 
 
<p/> 
<method name="void LHAup::listEvent()"> 
prints the above information for the current event.  In cases where the 
<code>LHAup</code> object is not available to the user, the 
<code>Pythia::LHAeventList()</code> method can 
be used, which is a wrapper for the above. 
</method> 
 
<method name="virtual bool LHAup::skipEvent(int nSkip)"> 
skip ahead <code>nSkip</code> events in the Les Houches generation 
sequence, without doing anything further with them. Mainly 
intended for debug purposes, e.g. when an event at a known 
location in a Les Houches Event File is causing problems. 
Will return false if operation fails, specifically if the 
end of an LHEF has been reached. The implementation in the base class 
simply executes <code>setEvent()</code> the requested number of times. 
The derived <code>LHAupLHEF</code> class (see below) only uses the 
<code>setNewEventLHEF(...)</code> part of its <code>setEvent()</code> 
method, and other derived classes could choose other shortcuts. 
</method> 
 
<p/> 
The LHA expects the decay of resonances to be included as part of the 
hard process, i.e. if unstable particles are produced in a process then 
their decays are also described. This includes <ei>Z^0, W^+-, H^0</ei> 
and other short-lived particles in models beyond the Standard Model. 
Should this not be the case then PYTHIA will perform the decays of all 
resonances it knows how to do, in the same way as for internal processes. 
Note that you will be on slippery ground if you then restrict the decay of 
these resonances to specific allowed channels since, if this is not what 
was intended, you will obtain the wrong cross section and potentially the 
wrong mix of different event types. (Since the original intention is 
unknown, the cross section will not be corrected for the fraction of 
open channels, i.e. the procedure used for internal processes is not 
applied in this case.) 
 
<p/> 
Even if PYTHIA can select resonance decay modes according to its 
internal tables, there is normally no way for it to know which 
decay angular correlations should exist in the simulated process. 
Therefore almost all decays are isotropic. The exceptions are Higgs and 
top decays, in the decay chains <ei>H &rarr; WW/ZZ &rarr; f fbar f' fbar'</ei> 
and <ei>t &rarr; b W &rarr; b f fbar</ei>, where the process-independent 
correlations implemented for internal processes are used. If part of 
the decay chain has already been set, however (e.g. <ei>H &rarr; WW/ZZ</ei> 
or <ei>t &rarr; b W</ei>), then decay is still isotropic. 
 
<p/> 
The LHA standard only allows for one hard subcollision in an event. 
Further multiparton interactions are supposed to be handled by the 
internal MPI machinery. As a nonstandard feature, it is possible 
to input two hard subcollisions in the same event, to match the internal 
<aloc href="SecondHardProcess">second hard process</aloc> machinery. 
In such cases two partons are extracted from each of the two incoming 
hadrons. A restriction is that, unlike the single-subprocess case, 
it is important that the partons are input in the order that PYTHIA 
later would need. That is, the two subcollisions should follow each 
other, with instate preceding outstate. Any resonance decay chain 
should be put at the end, after both interactions. As illustration, 
consider double <ei>W</ei> production. With <ei>1</ei> and <ei>2</ei> 
labelling the two subcollisions, and <ei>A</ei> and <ei>B</ei> the two 
incoming hadron beams, the event record ordering should be 
<ei>in_A1 - in_B1 - W_1 - in_A2 - in_B2 - W_2 - f_1 - fbar_1 - f_2 - 
fbar_2</ei>, where <ei>f fbar</ei> is the fermion decay products of 
the respective <ei>W</ei>. A limitation is that currently only one 
input scale is used, that thereby limits all further partonic activity 
in the same way for both processes. 
 
<p/> 
When transferring events through the runtime interface, it is worth 
noting that PYTHIA offers some settings to ensure the consistency of 
momenta, e.g. reshuffling of particle momenta to guarantee on-shell 
conditions,  or matching the sum of incoming to the sum of outgoing momenta. 
The documentation of settings related to this can be found under the 
<ei>Transfer to the PYTHIA process record</ei> heading of 
<aloc href="LHEF">Les Houches Event Files </aloc>. 
 
<h3>A LHEF reader class</h3> 
 
The LHEF standard (<ref>Alw06</ref>, <ref>But14</ref>) specifies a format 
where a single file packs initialization and event information. This has 
become the most frequently used procedure to process external parton-level 
events in Pythia, and is discussed in detail in the 
<aloc href="LHEF">Les Houches Event Files </aloc> section. 
Internally the file handling and reading is us handled by instance of the 
derived class <code>LHAupLHEF</code>. 
 
<p/> 
The workhorses of the <code>LHAupLHEF</code> class are three methods 
found in the base class, so as to allow them to be reused in other 
contexts. 
 
<method name="bool LHAup::setInitLHEF(ifstream& is, 
bool readHeaders = false)"> 
read in and set all required initialization information from the 
specified stream. With second argument true it will also read and store 
header information, as described above. Return false if it fails. 
</method> 
 
<method name="bool LHAup::setNewEventLHEF(ifstream& is)"> 
read in event information from the specified stream into a staging area 
where it can be reused by <code>setOldEventLHEF</code>. 
</method> 
 
<method name="bool LHAup::setOldEventLHEF()"> 
store the event information from the staging area into the normal 
location. Thus a single <code>setNewEventLHEF</code> call can be 
followed by several <code>setOldEventLHEF</code> ones, so as to 
process the same configuration several times. This method currently 
only returns true, i.e. any errors should be caught by the preceding 
<code>setNewEventLHEF</code> call. 
</method> 
 
<p/> 
These three main methods build on a number of container classes and a 
generic LHEF reader class (called <code>Reader</code>) found in 
<code>LHEF3.h</code> and <code>LHEF3.cc</code>. The <code>Reader</code> 
handles all the parsing and storage necessary to adhere with 
<ref>But14</ref>. (A matching <code>Writer</code> class is also 
available; see documentation in <code>LHEF3.h</code> how it can be 
used.) All parsing that is not strictly part of the LHEF format 
(e.g. the reading of header information) is instead performed directly in 
the <code>LHAupLHEF</code> methods. 
 
<p/> 
Some other small utility routines are: 
 
<method name="bool LHAup::fileFound()"> 
always returns true in the base class, but in <code>LHAupLHEF</code> 
it returns false if the LHEF provided in the constructor is not 
found and opened correctly. 
</method> 
 
<method name="bool LHAup::useExternal()"> 
always returns false in the base class, but in <code>LHAupLHEF</code> 
it returns false if the <code>LHAupLHEF</code> instance is constructed to 
work on an input LHE file, while it returns true if the <code>LHAupLHEF</code> 
instance is constructed to use externally provided input streams instead. 
For the latter, the <code>LHAupLHEF</code> instance should have been 
constructed with the class constructor 
<code>LHAupLHEF(Info* infoPtrIn, istream* isIn, istream* isHeadIn, 
bool readHeadersIn, bool setScalesFromLHEFIn)</code>. 
</method> 
 
<method name="void LHAup::setInfoHeader(const string &key, 
const string &val)"> 
is used to send header information on to the <code>Info</code> class. 
</method> 
 
<p/> 
A few other methods, most of them derived from the base class, 
streamlines file opening and closing, e.g. if several LHE files are 
to be read consecutively, without the need for a complete 
reinitialization. This presupposes that the events are of the same 
kind, only split e.g. to limit file sizes. 
 
<method name="bool LHAup::newEventFile(const char* fileIn)"> 
close current event input file/stream and open a new one, to 
continue reading events of the same kind as before. 
</method> 
 
<method name="istream* LHAup::openFile(const char *fn, ifstream &ifs)"> 
</method> 
<methodmore name="void LHAup::closeFile(istream *&is, ifstream &ifs)"> 
open and close a file, also gzip files, where an intermediate 
decompression layer is needed. 
</methodmore> 
 
<method name="void LHAupLHEF::closeAllFiles()"> 
close main event file (LHEF) and, if present, separate header file. 
</method> 
 
<h3>A runtime Fortran interface</h3> 
 
The runtime Fortran interface requires linking to an external Fortran 
code. In order to avoid problems with unresolved external references 
when this interface is not used, the code has been put in a separate 
<code>include/Pythia8Plugins/LHAFortran.h</code> file, that is not 
included in any of the other library files. Instead it should be included 
in the user-supplied main program, and used to create a derived class that 
contains the implementation of two methods below that call the Fortran 
program to do its part of the job. 
 
<p/> 
The <code>LHAupFortran</code> class derives from <code>LHAup</code>. 
It reads initialization and event information from the LHA standard 
Fortran commonblocks, assuming these commonblocks behave like two 
<code>extern "C" struct</code> named <code>heprup_</code> and 
<code>hepeup_</code>. (Note the final underscore, to match how the 
gcc compiler internally names Fortran files.) 
 
<p/> 
The instantiation does not require any arguments. 
 
<p/> 
The user has to supply implementations of the <code>fillHepRup()</code> 
and <code>fillHepEup()</code> methods, that is to do the actual calling 
of the external Fortran routines that fill the <code>HEPRUP</code> and 
<code>HEPEUP</code> commonblocks. The translation of this information to 
the C++ structure is provided by the existing <code>setInit()</code> and 
<code>setEvent()</code> code. 
 
<p/> 
Up to and including version 8.125 the <code>LHAupFortran</code> class 
was used to construct a runtime interface to PYTHIA 6.4. This was 
convenient in the early days of PYTHIA 8 evolution, when this program 
did not yet contain hard-process generation, and the LHEF standard 
did not yet exist. Nowadays it is more of a bother, since a full 
cross-platform support leads to many possible combinations. Therefore 
this support has been removed, but can still be recuperated from 
previous code versions, in a reduced form up to version 8.176. 
 
<h3>A LHEF writer class</h3> 
 
The main objective of the <code>LHAup</code> class is to feed information 
from an external program into PYTHIA. It can be used to export information 
as well, however. The main documentation of these features is found in 
the <aloc href="LHEF">Les Houches Event Files </aloc> section. Nevertheless, 
we quickly summarize the main workhorse functions here. There are four 
routines in the base class that should be called in sequence to build up the 
proper file structure. 
 
<method name="bool LHAup::openLHEF(string filename)"> 
Opens a file with the filename indicated, and writes a header plus a brief 
comment with date and time information. 
</method> 
 
<method name="bool LHAup::initLHEF()"> 
Writes initialization information to the file above. Such information should 
already have been set with the methods described in the "Initialization" 
section above. 
</method> 
 
<method name="bool LHAup::eventLHEF(bool verbose = true)"> 
Writes event information to the file above. Such information should 
already have been set with the methods described in the "Event input" 
section above. This call should be repeated once for each event to be 
stored. By default the event information is lined up in columns. 
To save space, the alternative <code>verbose = false</code> only 
leaves a single blank between the information fields. 
</method> 
 
<method name="bool LHAup::closeLHEF(bool updateInit = false)"> 
Writes the closing tag and closes the file. Optionally, if 
<code>updateInit = true</code>, this routine will reopen the file from 
the beginning, rewrite the same header as <code>openLHEF()</code> did, 
and then call <code>initLHEF()</code> again to overwrite the old 
information. This is especially geared towards programs, such as PYTHIA 
itself, where the cross section information is not available at the 
beginning of the run, but only is obtained by Monte Carlo integration 
in parallel with the event generation itself. Then the 
<code>setXSec( i, xSec)</code>, <code>setXErr( i, xSec)</code> and 
<code>setXMax( i, xSec)</code> can be used to update the relevant 
information before <code>closeLHEF</code> is called. 
<note>Warning:</note> overwriting the beginning of a file without 
upsetting anything is a delicate operation. It only works when the new 
lines require exactly as much space as the old ones did. Thus, if you add 
another process in between, the file will be corrupted. 
</method> 
 
<method name="string LHAup::getFileName()"> 
Return the name of the LHE file above. 
</method> 
 
</chapter> 
 
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