A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods such as the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree–Fock, and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as it is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite such as GAMESS is maintained and developed is considered. Future developments are briefly summarized.

The performance of 24 density functionals, including 14 meta-generalized gradient approximation (mGGA) functionals, is assessed for the calculation of vertical excitation energies against an experimental benchmark set comprising 14 small-to medium-sized compounds with 101 total excited states. The experimental benchmark set consists of singlet, triplet, valence, and Rydbergexcited states. The global-hybrid (GH) version of the Perdew-Burke-Ernzerhoff GGA density functional (PBE0) is found to offer the best overall performance with a mean absolute error (MAE) of 0. The performance of 24 density functionals, including 14 meta-generalized gradient approximation (mGGA) functionals, is assessed for the calculation of vertical excitation energies against an experimental benchmark set comprising 14 small-to medium-sized compounds with 101 total excited states. The experimental benchmark set consists of singlet, triplet, valence, and Rydberg excited states. The global-hybrid (GH) version of the Perdew-Burke-Ernzerhoff GGA density functional (PBE0) is found to offer the best overall performance with a mean absolute error (MAE) of 0.28 eV. The GH-mGGA Minnesota 2006 density functional with 54% Hartree-Fock exchange (M06-2X) gives a lower MAE of 0.26 eV, but this functional encounters some convergence problems in the ground state. The local density approximation functional consisting of the Slater exchange and Volk-Wilk-Nusair correlation functional (SVWN) outperformed all non-GH GGAs tested. The best pure density functional performance is obtained with the local version of the Minnesota 2006 mGGA density functional (M06-L) with an MAE of 0.41 eV.

The effects of solvents on electronic spectra can be treated efficiently by combining an accurate quantum mechanical (QM) method for the solute with an efficient and accurate method for the solvent molecules. One of the most sophisticated approaches for treating solvent effects is the effective fragment potential (EFP) method. The EFP method has been interfaced with several QM methods, including configuration interaction, time-dependent density functional theory, multiconfigurational methods, and equations-of-motion coupled cluster methods. These combined QM-EFP methods provide a range of efficient and accurate methods for studying the impact of solvents on electronic excited states. An energy decomposition analysis in terms of physically meaningful components is presented in order to analyze these solvent effects. Several factors that must be considered when one investigates solvent effects on electronic spectra are discussed, and several examples are presented. Disciplines Chemistry CommentsReprinted (adapted) hile multiple absorption and emission spectroscopic experimental studies provide valuable information on the magnitude and dynamics of soluteÀsolvent coupling, calculations on electronic excited states in the condensed phase remain a major challenge to the theoretical chemistry community.1 The increased number of nuclear and electronic degrees of freedom relative to the gas phase makes accurate fully ab initio calculations on a condensed-phase system unfeasible long before the system can approach the bulk. One general approach to this type of problem is to separate a system into two parts, such that one (active, usually solute) part is treated by quantum mechanical (QM) techniques and the other (usually larger, solvent) part is calculated using classical (molecular) mechanics (MM).2 The Hamiltonian of the system then consists of three termŝIn eq 1, H QM/MM is a coupling term between the two levels of theory. Separation of the QM and MM subsystems, in principle, allows one to use any level of theory in both the QM and MM parts.There have been an increasing number of studies devoted to a description of electronic spectroscopy in the condensed phase.3À14 An alternative to the QM/MM approach is to study the electronic excited states of solutes with dielectric continuum methods. 3À5,9,11,12,15 While continuum models are computationally inexpensive, they cannot describe explicit solventÀsolute interactions such as hydrogen bonding. Another promising approach for studying condensed-phase electronic spectroscopy in large molecular systems is to use a fragment-based technique, such as the fragment molecular orbital method (FMO). 16À20The FMO and related methods have the advantage of being close to fully "ab initio", but these methods are still sufficiently computationally demanding that (for example) performing molecular dynamics simulations on excited states in solution is still not feasible.If one is to perform QM/MM calculations to accurately capture solvent effects on electronic excited states, it i...