skip to main content

SciTech ConnectSciTech Connect

Title: Configuration interaction singles natural orbitals: An orbital basis for an efficient and size intensive multireference description of electronic excited states

Multireference quantum chemical methods, such as the complete active space self-consistent field (CASSCF) method, have long been the state of the art for computing regions of potential energy surfaces (PESs) where complex, multiconfigurational wavefunctions are required, such as near conical intersections. Herein, we present a computationally efficient alternative to the widely used CASSCF method based on a complete active space configuration interaction (CASCI) expansion built from the state-averaged natural orbitals of configuration interaction singles calculations (CISNOs). This CISNO-CASCI approach is shown to predict vertical excitation energies of molecules with closed-shell ground states similar to those predicted by state averaged (SA)-CASSCF in many cases and to provide an excellent reference for a perturbative treatment of dynamic electron correlation. Absolute energies computed at the CISNO-CASCI level are found to be variationally superior, on average, to other CASCI methods. Unlike SA-CASSCF, CISNO-CASCI provides vertical excitation energies which are both size intensive and size consistent, thus suggesting that CISNO-CASCI would be preferable to SA-CASSCF for the study of systems with multiple excitable centers. The fact that SA-CASSCF and some other CASCI methods do not provide a size intensive/consistent description of excited states is attributed to changes in the orbitals that occur upon introduction ofmore » non-interacting subsystems. Finally, CISNO-CASCI is found to provide a suitable description of the PES surrounding a biradicaloid conical intersection in ethylene.« less
Authors:
;  [1] ;  [2]
  1. Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (United States)
  2. Department of Chemistry, City College of New York, New York, New York 10031 (United States)
Publication Date:
OSTI Identifier:
22415818
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 142; Journal Issue: 2; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CONFIGURATION INTERACTION; ELECTRON CORRELATION; ETHYLENE; EXCITATION; EXCITED STATES; GROUND STATES; MOLECULES; POTENTIAL ENERGY; SELF-CONSISTENT FIELD; SURFACES; VARIATIONAL METHODS