Showing papers by "Ernest R. Davidson published in 2014"
TL;DR: A systematic sequence of configuration interaction and coupled cluster calculations were used to describe selected low-lying singlet and triplet vertically excited states of ethylene with the goal of approaching the all electron, full configuration interaction/complete basis set limit.
Abstract: A systematic sequence of configuration interaction and coupled cluster calculations were used to describe selected low-lying singlet and triplet vertically excited states of ethylene with the goal of approaching the all electron, full configuration interaction/complete basis set limit. Included among these is the notoriously difficult, mixed valence/Rydberg 1B1u V state. Techniques included complete active space and iterative natural orbital configuration interaction with large reference spaces which led to variational spaces of 1.8 × 109 parameters. Care was taken to avoid unintentionally biasing the results due to the widely recognized sensitivity of the V state to the details of the calculation. The lowest vertical and adiabatic ionization potentials to the 2B3u and 2B3 states were also determined. In addition, the heat of formation of twisted ethylene 3A1 was obtained from large basis set coupled cluster theory calculations including corrections for core/valence, scalar relativistic and higher order correlation recovery.
38 citations
TL;DR: The new ROHF-DODS method is developed which combines the use of DODS underlying amended UHF methods and the main advantage of the canonical ROHf method which is a fulfillment of the rigorous Koopmans' conditions.
Abstract: This work compares different approaches to deriving Hartree-Fock (HF) orbitals and orbital energies for open-shell systems. We compare the basic HF equations underlying both the classic open-shell HF methods, which are the restricted open-shell HF (ROHF) and unrestricted HF (UHF) methods, and a number of the novel (amended) versions of these methods. The main attention is paid to a treatment of the validity of Brillouin's and Koopmans’ theorems in the amended versions. We show that these two theorems are fully obeyed only in the special (canonical) form of the ROHF method developed by Plakhutin et al. [J. Chem. Phys. 125, 204110 (2006)] and by Davidson and Plakhutin [J. Chem. Phys. 132, 184110 (2010)], while each of the amended UHF methods suffers from some deficiencies inherent to original UHF and ROHF methods. To compare the HF orbitals derived by different methods in two different forms – DODS (different orbitals for different spins) and SODS (the same orbitals for different spins), we develop the new ...
19 citations