New features for V2.1 (for the features of V2.0X relative to V1.X, see the version-update history (V2.01->V2.06):

Makefiles, compilation:
To help the user select the compilers and compilation options most suitable to his platform, these are now collected in a single file located in the main directory: compile.mk

The Makefile for the various processes refer to compile.mk for the compiler options, and to alplib/alpgen.mk for the list of files to be compiled. The executable can now be generated within each working directory issuing the command:

• For the F77 version: > make gen
• For the F90 version: > make gen90

Updates to jet-parton matching:
The use of jet-parton matching is described in the version history of V2.0 . A complete review of the MLM matching scheme is being documented in a forthcoming publication.

Few changes to the matching are implemented in V2.1:

1. The matching and vetoing for events with heavy quarks (charm, b and top) has been improved. For these processes the matching now proceeds in two steps. First the matching of the light partons with the jets is performed. As in previous versions, radiation off the heavy quarks is not used to cluster the jets used in the matching. The radiation emitted by the heavy quarks is then subsequently included in the recostruction of possible additional jets, and is used to veto against extra-jet emissions when generating exclusive samples.
2. The treatment of the highest-multiplicity jet sample, which is to be generated in "inclusive mode (iexc=0) has been slightly modified. Events with extra jets are kept only if the jets which do not match partons have pt smaller than the minimum pt of the matched jets.
3. The default parameters of the matching algorithm (the minimum et of the clusters, their rapidity range) have been modified. The proposed defaults appear to be better than the previous ones.
4. The scale choice has been slightly modified. In the case of processes that at LO start with several powers of alphas (like jet production, 2Q  or wqq), we separate the powers of alphas that accompany the LO process from those that accompany the additional light partons. It is only to these extra powers that the CKKW scale is applied. By construction, this means that for a LO process now ickkw=0 or =1 should give the same result as ickkw=0 before.
   
5. The matching algorithm for some processes (most notably vbjet with vector boson fusion) is still being optimzied, and will likely be updated in forthcoming releases
   

Other notable features: 

• A photon-photon minimal separation parameter (drphmin) has been introduced. So far the minimal separation between photon pairs was taken to be equal to the minimal dR separaiotn between light jets. This, among other changes, called for an addition to the alpgen.inc common blocks, which are therefore now incompatible with those of V2.0X.
• The default value of the charm mass has been set to 1.5 gev (was 0) for all processes with an explicit final-state charm (e.g. Wc cbar+jets, Wc+jets, cc+jets, etc)
• In the process Wc, it is now possible to place separate generation cuts for the charm (ptcmin and etacmax) and for the light jets (ptjmin, etajmax).
  

Remarks/Bugs for V2.1. This section contains comments and/or bug reports. The source library linked above contains the most up-to-date version, with all bugs reported below fixed. It is advised to check this section from time to time, to ensure you are running the most up-to-date version. Only the program elements explicitly listed have been updated with respect to the previous version.

• Fixed a bug that disabled the proper application of matching after the shower evolution of the 2Qph processes (associated production of heavy quarks, a photon ad jets).
• Fixed a few compatibility issues relative to the latest versions of the gfortran compiler
• Added new sets of PDFs:
  • MSTW2008: LO, NLO, LO68CL, LO*, LO**
  • Ctq66, CT09MC1, CT09MC2
  Run the code interactively in imode=3 to display the full list and the relative PDF codes. The directory structure for the PDF data files was modified, but this is transparent for the user.
• Introduced new input parameters xlclu and lpclu (respectively lambda and nloop for the evaluation of alphas using the CKKW scale). By default they are equal to the Lambda_QCD and nloop values of the selected PDF set. They are now allowed to vary, to better match the choice of alpha_s made by the MC used for the shower evolution.
• Introduced a new scale choice for the wqq, zqq, wjet, zjet processes: iqopt=5 will select Q=HT, the sum of transverse momenta of all jets plus the decay products of W and Z bosons.