Excitations in photoactive molecules from quantum Monte Carlo

Abstract

Despite significant advances in electronic structure methods for the treatment of excited states, attaining an accurate description of the photoinduced processes in photoactive biomolecules is proving very difficult. For the prototypical photosensitive molecules, formaldimine, formaldehyde, and a minimal protonated Schiff base model of the retinal chromophore, we investigate the performance of various approaches generally considered promising for the computation of excited potential energy surfaces. We show that quantum Monte Carlo can accurately estimate the excitation energies of the studied systems if one constructs carefully the trial wave function, including in most cases the reoptimization of its determinantal part within quantum Monte Carlo. While time-dependent density functional theory and quantum Monte Carlo are generally in reasonable agreement, they yield a qualitatively different description of the isomerization of the Schiff base model. Finally, we find that the restricted open shell Kohn-Sham method is at variance with quantum Monte Carlo in estimating the lowest-singlet excited state potential energy surface for low-symmetry molecular structures.

Excited state calculations using phaseless auxiliary-field quantum Monte Carlo: Potential energy curves of low-lying C2 singlet states

“…In this subsection, we present phaseless AFQMC/CASSCF PECs with large basis sets and compare with experimental spectroscopic values. Figure 5 shows AFQMC/CASSCF PECs for the three lowest lying singlet states, using truncated CASSCF (8,16) wave function as |Φ r , in the cc-pVTZ basis set. 26 For comparison, the figure also shows the RCCSD(T) ground state PEC, which was calculated using the NWCHEM 34 computer program.…”

“…As described in Sec. II C, these are truncated CASSCF (8,16) wave functions, with no further optimization. Figure 4 compares the AFQMC/CASSCF and FCI calculated PECs.…”

“…Most of our AFQMC calculations use as |Φ r a truncated CASSCF (8,16) wave function, obtained from the GAMESS quantum chemistry program. 32 The CASSCF wave function is truncated such that the weight (squared coefficient) of the retained determinants is ∼ 97% of the total.…”

“…3,4 The most commonly applied quantum Monte Carlo (QMC) method in electronic structure has been diffusion Monte Carlo (DMC), 5 which has also been used to compute excited states. 6,7 Compared to ground states, however, the accuracy of the results may depend on the symmetry 6 and show greater sensitivity 8 to the trial wave function used in the fixed-node approximation to control the sign problem and maintain orthogonality.…”

Numerical Methods for Electronic Structure Calculations of Materials

“…The Car-Parrinello Molecular Dynamics Consortium website [172] lists numerous papers that have been published on this method since 1994; however, this list is not exhaustive. To give an idea of the wide range of applications studied by this method, we list several papers in materials science [32,78,106,109,185], physics [14,20,46,143,154], chemistry [23,57,87,137,184], and biology [56,80,113,132,146] that employ this method. For information on recent advances in chemistry and materials science with Car-Parinello molecular dynamics methods, see [1].…”