Analytic energy gradients for open-shell coupled-cluster singles and doubles (CCSD) calculations using restricted open-shell Hartree—Fock (ROHF) reference functions
✍ Scribed by Jürgen Gauss; Walter J. Lauderdale; John F. Stanton; John D. Watts; Rodney J. Bartlett
- Publisher
- Elsevier Science
- Year
- 1991
- Tongue
- English
- Weight
- 768 KB
- Volume
- 182
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
The theory of analytic energy gradients for the open-shell single and double excitation coupled-cluster (CCSD) method based on restricted open-shell Hartrce-Fock (ROHF) reference functions is presented. The new CCSD gradient method is applied to the dissociation of the 'A" state of formaldehyde (CH,O) to H and HCO. Complete geometry optimization is carried out for the initial reactant, the transition state of the reaction, and for the dissociation products. Non-iterative triples appropriate to ROHF are introduced and are used to define a new ROHF-CCSD(T) method. Using this approach and zero-point corrections, the activation energy is calculated to be 21 .O kcaljmol and the exit barrier height is predicted to be 6.1 kcal/mol, both in excellent agreement with experiment.
'I For a review of CC methods, see ref. [ 11. '* This is only true for the "standard CC" methods which are obtained by projecting the Schrijdinger equation on the reference function and excited determinants. The situation is different with respect to CC methods based on an expectation value ansatz such as the recently implemented unitary CC (UCC(4)) approach (see for example ref. [I I I).