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On the use of reduced active space in CASSCF calculations

2014; Elsevier BV; Volume: 1040-1041; Linguagem: Inglês

10.1016/j.comptc.2014.03.017

2210-2728

Martial Boggio‐Pasqua, Gerrit Groenhof,

Quantum, superfluid, helium dynamics

• We propose some guidelines to reduce the active space size in CASSCF calculations . • We reinvestigated the benzene to benzvalene S 1 photochemical pathway. • (6e, 6o) active space is required for benzene S 1 photochemistry . • We studied the photoisomerization of the PYP chromophore . • Reliable results can be obtained with a reduced (10e, 9o) active space. The complete active space self-consistent field (CASSCF) method is one of the most used approaches to explore theoretically non-adiabatic photochemical processes in medium-size molecular systems. It is also frequently utilized in direct dynamics studies because this method offers a reliable description of the electronic state mixing in the regions of surface crossings (conical intersections). In this context, however, the large computational demand required to perform such on-the-fly molecular dynamics simulations using CASSCF as the electronic structure method means that only relatively small active space can be handled (typically not more than 10 electrons distributed in 10 orbitals). In this article, we present some guidelines to reduce the active space size in CASSCF calculations in order to decrease the computational cost necessary to perform, for example, molecular dynamics simulations. Three important criteria are considered: (i) the degradation of the accuracy in the description of the potential energy surface following the reduction of the active space, (ii) the stability of the reduced active space across the global potential energy surface, and (iii) the cost of the CASSCF calculation using the reduced active space. Two illustrations are given. The first one is the benzene to benzvalene S 1 photochemical pathway, the second one is the photoisomerization of thiomethyl para -coumaric acid, the chromophore in photoactive yellow protein.