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Abstract
In this paper we discuss a method to obtain approximate natural orbitals (ANOs) by means of the zero-order and first-order one-particle reduced density matrices. Convergence of the multireference second-order perturbation CI by means of CIPSI method, in its two-class and three-class versions, appears to be much improved when using these ANOs. The method requires only a single additional integral transformation. A comparison with canonical and valence virtual orbitals defined through the PHO procedure is also reported. Finally, the use of ANOs together with the multireference analog of the Davidson’s correction, as well as a suitable extrapolation procedure in the CIPSI method is discussed. Results are compared with available full CI calculations for H2O in a double-ξ and a double-ξ plus polarization basis set.
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