Assessment of semiempirical methods for the photoisomerisation of a protonated Schiff base

Abstract

The potential energy surfaces and non-adiabatic dynamics of the C 5 H 6 NH 2 ? protonated Schiff base (PSB3) have been investigated using the OM2 semiempirical Hamiltonian with GUGA configuration interaction. Three approaches to selecting the GUGA-CI active space are evaluated using closed-shell and open-shell molecular orbitals. Energy minima and minimum energy crossing points (MECPs) have been compared with ab initio CASSCF and CASPT2 results. Only the open-shell calculations give a qualitatively correct MECP. Minimum energy path (MEP) calculations demonstrate that a minimal active space gives a barrierless path from the planar S 1 minimum to the ground state, whereas larger active spaces result in a small barrier to torsional motion. Surface hopping dynamics calculations indicate that this barrier induces bi-exponential dynamics. The comparable CAS-SCF S 1 energy surface is barrierless, but the CASPT2 surface features an energy plateau, which may also lead to more complex dynamics.

Assessment of Density Functional Methods for Obtaining Geometries at Conical Intersections in Organic Molecules

“…Further detail on the reference calculations can be found in the original publications. 25,26 For HDI, only the CI with phenyl ring torsion was studied using the CASPT2 method; 25 however, both CIs, CI Ph (Scheme 1I) and CI Im (Scheme 1J), were investigated by the CASSCF method. 52 Because the latter approach does not systematically include the dynamic electron correlation, the CASSCF results are not used when benchmarking the density functional calculations.…”

“…The performance of several popular density functionals will be investigated in comparison with the results of high-level ab initio multireference wave function calculations available in the literature. 25,26 The results of density functional calculations will be analyzed in chemical terms, where the elementary reaction coordinates of photorearrangement will be employed. 27 In section 2 the properties of conical intersection points and the method of calculation will be briefly outlined.…”

On the effect of a variation of the force field, spatial boundary condition and size of the QM region in QM/MM MD simulations

“…OM3, a simplified and reparametrized version of OM2, includes corrections to the two-center core Hamiltonian matrix elements to introduce a dependence on the molecular environment by three-center terms and an alternative effective core potential to account for core-valence orthogonalization. It has been reported that the orthogonalization corrections improve the description of orbital-energy patterns, [49,53] electronic excitation energies, [46,53,54] closed shell repulsions, [46] conformational preferences [55] and hydrogen-bonding interactions. [46,56] The improved description of hydrogen-bonding has also been confirmed in combined QM/MM calculations on hydrogen-bonding complexes.…”

Isomerization mechanism of the HcRed fluorescent protein chromophore

“…To always retain the p orbitals in the active space, we used a recently developed method for identifying and tracking the p character of orbitals (applying a threshold of 0.4 for the sum of the local p populations). 86 In these tests, the calculated excitation energies did not change significantly upon increasing the size of the active space (see below). Hence, the smallest active space (10, 9) was chosen as standard for the OM2-MRCI excited-state calculations, both under vacuum and in HcRed.…”