The closed-shell CCSD equations are reformulated in order to achieve superior computational efficiency. Using a spin adaptation scheme based on the unitary group approach (UGA), we have obtained a new set of equations that greatly improves our previous formulation. Based on this scheme we have also derived equations for the closed-shell configuration interaction including all single and double excitations (CISD) case. Both methods have been implemented and tested. For a range of test cases the new CCSD method is more efficient than the earlier CCSD method. The new closed-shell CISD procedure is faster than the shape-driven (SD)GUGA algorithm and the new CCSD scheme is less than two times more computation intensive than SDGUGA CISD per iteration.
The PSI4 program is a new approach to modern quantum chemistry, encompassing Hartree-Fock and density-functional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit high-efficiency computations of both standard and emerging electronic structure methods on conventional and high-performance parallel computer architectures. PSI4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the program's capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, PSI4 is available through an open-source license to the entire scientific community.
For comparison with experimentally obtained thermochemical data, zero-point vibrational energies (ZPVEs) are required to convert total electronic energies obtained from ab initio quantum mechanical studies into 0 K enthalpies. The currently accepted practice is to employ self-consistent-field (SCF) harmonic frequencies that have been scaled to reproduce experimentally observed fundamental frequencies. This procedure introduces systematic errors that result from a recognizable flaw in the method, namely that the correct ZPVE, G(0), is not one half the sum of the fundamental vibrational frequencies. Until better methods for accurately determining ZPVEs are presented, we recommend using different scaling factors for the determination of ZPVEs than those used to compare theoretically determined harmonic frequencies to observed fundamentals.
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