Coupled Cluster Methods for Multi-Reference Applications
Abstract
Report developed under STTR contract for AF09-BT40. In Phase I we have studied several different approaches to the multi-reference coupled-cluster (MR-CC) problem, all the existing ones and introduced three new ones. The latter are the Tailored CC, (TCCSD(T)), the multi-reference double ionized and double electron attached (MR-DI/DA-CC), and a new adapted method,(@CC) which guarantees accurate results for single or multi-reverence problems and for ground or excited states. We believe these offer better alternatives for wide spread application to molecular structure and spectra. All have been compared on examples including the auto-isomerization of cyclobutadiene from its rectangular form through its square transition state, back to its other rectangular form. This example is notable as it has one of the largest numerical effects seen for multi-reference character measured as the difference between the single-reference results given by CCSD(T) and MR-CC. The correct answer for the barrier is expected to be <10 kcal/mol yet CCSD(T) gives 18 kcal/mol. TCCSD(T) gives 7 kcal/mol and MR-D1-CCSD gives 10 kcal/mol. We also made applications of the latter to several different potential energy curves to demonstrate that unlike SR-CC, MR-DI-CCSD provides correct separation all the way to the separated atom limit in a purely symmetry adapted way. This opens a new, attractive route to most MR problems.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 11, 2011
- Accession Number
- ADA545028
Entities
People
- Mark Ponton
- Rodney J. Bartlett