81 research outputs found
Relativistic Internally Contracted Multireference Electron Correlation Methods
We report internally contracted relativistic multireference configuration
interaction (ic-MRCI), complete active space second-order perturbation
(CASPT2), and strongly contracted n-electron valence state perturbation theory
(NEVPT2) on the basis of the four-component Dirac Hamiltonian, enabling
accurate simulations of relativistic, quasi-degenerate electronic structure of
molecules containing transition-metal and heavy elements. Our derivation and
implementation of ic-MRCI and CASPT2 are based on an automatic code generator
that translates second-quantized ansatze to tensor-based equations, and to
efficient computer code. NEVPT2 is derived and implemented manually. The
rovibrational transition energies and absorption spectra of HI and TlH are
presented to demonstrate the accuracy of these methods
On-the-fly CASPT2 surface hopping dynamics
We report the development of programs for on-the-fly surface hopping dynamics
simulations in the gas and condensed phases on the potential energy surfaces
computed by multistate multireference perturbation theory (XMS-CASPT2) with
full internal contraction. On-the-fly nonadiabatic dynamics simulations are
made possible by improving the algorithm for XMS-CASPT2 nuclear energy gradient
and derivative coupling evaluation. The program is interfaced to a surface
hopping dynamics program, Newton-X, and a classical molecular dynamics package,
tinker, to realize such simulations. On-the-fly XMS-CASPT2 surface-hopping
dynamics simulations of 9H-adenine and an anionic GFP model chromophore
(para-hydroxybenzilideneimidazolin-5-one) in water are presented to demonstrate
the applicability of our program to sizable systems. Our program is implemented
in the bagel package, which is publicly available under the GNU General Public
License
On the accuracy of retinal protonated Schiff base models
We investigate the molecular geometries of the ground state and the minimal
energy conical intersections (MECIs) between the ground and first excited
states of the models for the retinal protonated Schiff base in the gas phase
using the extended multistate complete active space second-order perturbation
theory (XMS-CASPT2). The biggest model in this work is the rhodopsin
chromophore truncated between the {\epsilon} and {\delta} carbon atoms, which
consists of 54 atoms and 12-orbital {\pi} conjugation. The results are compared
with those obtained by the state-averaged complete active space self-consistent
field (SA-CASSCF). The XMS-CASPT2 results suggest that the minimum energy
conical intersection associated with the so-called 13-14 isomerization is
thermally inaccessible, which is in contrast to the SA-CASSCF results. The
differences between the geometries of the conical intersections computed by
SA-CASSCF and XMS-CASPT2 are ascribed to the fact that the charge transfer
states are more stabilized by dynamical electron correlation than the
diradicaloid states. The impact of the various choices of active spaces, basis
sets, and state averaging schemes is also examined.Comment: Contribution to the special issue in honor of the 80th birthday of
Professor Michael Bae
Analytical derivative coupling for multistate CASPT2 theory
The probability of non-radiative transitions in photochemical dynamics is
determined by the derivative couplings, the couplings between different
electronic states through the nuclear degrees of freedom. Efficient and
accurate evaluation of the derivative couplings is, therefore, of central
importance to realize reliable computer simulations of photochemical reactions.
In this work, the derivative couplings for multistate multireference
second-order perturbation theory (MS-CASPT2) and its 'extended' variant
(XMS-CASPT2) are studied, in which we present an algorithm for their analytical
evaluation. The computational costs for evaluating the derivative couplings are
essentially the same as those for calculating the nuclear energy gradients. The
geometries and energies calculated with XMS-CASPT2 for small molecules at
minimum energy conical intersections (MECIs) are in good agreement with those
computed by multireference configuration interaction. As numerical examples,
MECIs are optimized using XMS-CASPT2 for stilbene and a GFP model chromophore
(the 4-para-hydroxybenzylidene-1,2-dimethyl-imidazolin-5-one anion)
Occupied-orbital fast multipole method for efficient exact exchange evaluation
We present an efficient algorithm for computing the exact exchange
contributions in the Hartree-Fock and hybrid density functional theory models
on the basis of the fast multipole method (FMM). Our algorithm is based on the
observation that FMM with hierarchical boxes can be efficiently used in the
exchange matrix construction, when at least one of the indices of the exchange
matrix is constrained to be an occupied orbital. Timing benchmarks are
presented for alkane chains (C400H802 and C150H302), a graphene sheet
(C150H30), a water cluster [(H2O)100], and a protein Crambin
(C202H317O64N55S6). The computational cost of the far-field exchange evaluation
for Crambin is roughly 3% that of a self-consistent field iteration when the
multipoles up to rank 2 are used
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