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Extended Dynamically Weighted CASPT2: The Best of Two Worlds
journal contribution
posted on 2020-02-21, 15:44 authored by Stefano Battaglia, Roland LindhWe
introduce a new variant of the complete active space second-order
perturbation theory (CASPT2) method that performs similarly to multistate
CASPT2 (MS-CASPT2) in regions of the potential energy surface where
the electronic states are energetically well separated and is akin
to extended MS-CASPT2 (XMS-CASPT2) in case the underlying zeroth-order
references are near-degenerate. Our approach follows a recipe analogous
to that of XMS-CASPT2 to ensure approximate invariance under unitary
transformations of the model states and a dynamic weighting scheme
to smoothly interpolate the Fock operator between state-specific and
state-average regimes. The resulting extended dynamically weighted
CASPT2 (XDW-CASPT2) methodology possesses the most desirable features
of both MS-CASPT2 and XMS-CASPT2, that is, the ability to provide
accurate transition energies and correctly describe avoided crossings
and conical intersections. The reliability of XDW-CASPT2 is assessed
on a number of molecular systems. First, we consider the dissociation
of lithium fluoride, highlighting the distinctive characteristics
of the new approach. Second, the invariance of the theory is investigated
by studying the conical intersection of the distorted allene molecule.
Finally, the relative accuracy in the calculation of vertical excitation
energies is benchmarked on a set of 26 organic compounds. We found
that XDW-CASPT2, albeit being only approximately invariant, produces
smooth potential energy surfaces around conical intersections and
avoided crossings, performing equally well to the strictly invariant
XMS-CASPT2 method. The accuracy of vertical transition energies is
almost identical to MS-CASPT2, with a mean absolute deviation of 0.01–0.02
eV, in contrast to 0.12 eV for XMS-CASPT2.