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PC GAMESS/Firefly shares lots of functionality with the GAMESS(US). All the functionality of October 25, 1999 release of GAMESS (US) is supported. Many additional features added to GAMESS (US) since 1999 are supported as well. Ab initio SCF wavefunctions for RHF, UHF, ROHF, GVB and MCSCF cases are available. Correlation corrections include configuration interaction (CI) and Moller-Plesset (MP) perturbation theory. Excited electronic states can be described via the configuration interaction singles (CIS), RPA, and TDDFT approximations. Effective core potentials and a variety of density functional (DFT) approaches are available. Geometry optimization can be performed with analytical gradients at the HF, MP2, CI, and DFT levels. Analytical Hessian can be computed at RHF, ROHF and GVB wavefunctions while numerical Hessians are available for all methods that support analytic gradients.
PC GAMESS/Firefly extends functionality of the GAMESS(US) program by providing very fast Moller-Plesset second, third and fourth order correlation corrections based on the self-consistent electron pairs (SCEP) theory. It allows separate evaluation of singlet and triplet pair contributions to MP2, MP3, and MP4(D) energies. The DFT code in the PC GAMESS/Firefly is completely different and is much faster than in the GAMESS (US) and the two programs support different density functionals. PC GAMESS/Firefly includes support of RPA (TDHF) and TDDFT for excited states.
PC GAMESS/Firefly lacks some features that have been implemented in the recent versions of GAMESS (US). For example, PC GAMESS/Firefly currently does not support coupled cluster methods and the fragment molecular orbitals. Both PC GAMESS/Firefly and GAMESS (US) lack support for h and higher angular momentum basis functions.