Investigation of an asymmetric triple-excitation correction for coupled-cluster energies

✍ Scribed by T. Daniel Crawford; John F. Stanton

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 316 KB
• Volume: 70
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(1998)70:4/5<601::aid-qua6>3.0.co;2-z

✦ Synopsis

A correction for the effects of connected triple excitations to the coupled-cluster singles and doubles energy is studied. The approach relies on the fact that the ground-state coupled-cluster energy may be viewed as an eigenvalue of an Ž . effective similarity transformed Hamiltonian with associated left and right eigenvectors. Taking these as zeroth-order wave functions and using a conventional partitioning of the bare electronic Hamiltonian, the lowest order triple-excitation correction to the correlation energy is found to have an asymmetric form that involves cluster amplitudes as well as Ž . components of the left eigenvector. The popular T correction may be viewed as an approximation to the present approach, though the latter is approximately a factor of 2 more expensive to compute. The method is applied to a number of difficult cases, including the harmonic vibrational frequencies of ozone and the equilibrium bond length of N . In addition, the theory of analytic gradients for the method is outlined and some 2 aspects regarding its implementation are discussed.