Speaker
Piotr Piecuch
Description
SINGLE-REFERENCE COUPLED-CLUSTER METHODS FOR MULTI-REFERENCE
MOLECULAR PROBLEMS
Piotr Piecuch, Jun Shen, Nicholas P. Bauman, and Jared A. Hansen
Accurate modeling of chemical reactions and photochemistry
requires a balanced treatment of dynamical and non-dynamical
many-electron correlation effects. The popular
single-reference coupled-cluster (CC) and equation-of-motion
CC (EOMCC) methods, such as CCSD(T) and EOMCCSD, capture the
former effects very effectively, but have difficulties with
the latter ones, whereas multi-reference CC theories that
are supposed to capture both types of correlations continue
facing unresolved problems. This talk will discuss pragmatic
ways of addressing this situation via the completely
renormalized and active-space CC and EOMCC theories, and
their recent merger via the novel CC(P;Q) formalism, which
reproduces the nearly exact relative and total electronic
energies in ground and excited states at the small fractions
of computer costs of other methods that aim at similar
accuracies. The development of the singly and doubly ionized
and electron-attached variants of the active-space EOMCC
methodology, which provide an excellent description of
electronic excitations in radicals, biradicals, and other
similar open-shell systems around closed shells at the low
computational costs compared to the parent approaches, will
be addressed, too.
