Systematic Expansion of Active Spaces beyond the CASSCF Limit: A GASSCF/SplitGAS Benchmark Study

Abstract

The applicability and accuracy of the generalized active space self-consistent field, (GASSCF), and (SplitGAS) methods are presented. The GASSCF method enables the exploration of larger active spaces than with the conventional complete active space SCF, (CASSCF), by fragmentation of a large space into subspaces and by controlling the interspace excitations. In the SplitGAS method, the GAS configuration interaction, CI, expansion is further partitioned in two parts: the principal, which includes the most important configuration state functions, and an extended, containing less relevant but not negligible ones. An effective Hamiltonian is then generated, with the extended part acting as a perturbation to the principal space. Excitation energies of ozone, furan, pyrrole, nickel dioxide, and copper tetrachloride dianion are reported. Various partitioning schemes of the GASSCF and SplitGAS CI expansions are considered and compared with the complete active space followed by second-order perturbation theory, (CASPT2), and multireference CI method, (MRCI), or available experimental data. General guidelines for the optimum applicability of these methods are discussed together with their current limitations.

MCSCF optimization revisited. II. Combined first- and second-order orbital optimization for large molecules

“…However, the number of CSFs increases factorially with the number of active orbitals, and the largest calculations so far included 22 electrons in 22 active orbitals. 73 One way of tackling the exploding number of configuration is to divide the active space into subsets and to add occupation restrictions to each subspace, as for example in the restricted active space (RAS) 9,74 or the generalized active space (GAS) 75,76 methods. In addition to this, several approximate full CI (FCI) methods have recently been developed and integrated into CASSCF, as for example the FCI quantum Monte Carlo (FCIQMC) method, [77][78][79] the heat-bath CI, [80][81][82] or the density matrix renormalization group (DMRG) methods.…”

A general second order complete active space self-consistent-field solver for large-scale systems

“…However, due to the exponential scaling of FCI, it is limited to small complete active spaces (CAS), usually no more than CAS (16,16) (16 electrons in 16 orbitals). However, there are now several techniques which can be used to replace the FCI solver [19,20,21,22,23,24]. Two of the more commonly used ones are the density matrix renormalization group (DMRG) [19] and full configuration interaction quantum Monte Carlo (FCIQMC) [21,22].…”

Low-lying excited states of model proteins: Performances of the CC2 method versus multireference methods

“…Therefore, it will be necessary to consider other solutions to optimize the orbitals, the first step of the CAS+DDCI+Q calculations. A promising solution that we are investigating on capped peptides containing at least two residues is given by the Generalized Active Space Self Consistent Field (GASSCF) method, 53,54 a method which allows us to expand active spaces beyond the CASSCF limit. This work will be reported in a future paper.…”