3 research outputs found
Optimized Jastrow-Slater wave functions for ground and excited states: Application to the lowest states of ethene
A quantum Monte Carlo method is presented for determining multi-determinantal
Jastrow-Slater wave functions for which the energy is stationary with respect
to the simultaneous optimization of orbitals and configuration interaction
coefficients. The approach is within the framework of the so-called energy
fluctuation potential method which minimizes the energy in an iterative fashion
based on Monte Carlo sampling and a fitting of the local energy fluctuations.
The optimization of the orbitals is combined with the optimization of the
configuration interaction coefficients through the use of additional single
excitations to a set of external orbitals. A new set of orbitals is then
obtained from the natural orbitals of this enlarged configuration interaction
expansion. For excited states, the approach is extended to treat the average of
several states within the same irreducible representation of the pointgroup of
the molecule. The relationship of our optimization method with the stochastic
reconfiguration technique by Sorella et al. is examined. Finally, the
performance of our approach is illustrated with the lowest states of ethene, in
particular with the difficult case of the singlet 1B_1u state.Comment: 12 pages, 2 figure
Excitations in photoactive molecules from quantum Monte Carlo
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.Comment: 10 pages, 6 figure