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6.14 Non-Hartree-Fock Orbitals in Correlated Calculations

6.14.1 Overview

(April 13, 2024)

In cases of problematic open-shell references, e.g., strongly spin-contaminated doublet radicals, one may choose to use DFT orbitals, which can yield significantly improved results. 97 Beran G. J. O., Gwaltney S. R., Head-Gordon M.
Phys. Chem. Chem. Phys.
(2003), 5, pp. 2488.
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This can be achieved by first doing DFT calculation and then reading the orbitals and turning the SCF procedure off.

Example 6.38  CCSD calculation of triplet methylene using B3LYP orbitals

$molecule
   0 3
   C
   H  1  CH
   H  1  CH  2  HCH

   CH  = 1.07
   HCH = 111.0
$end

$rem
   EXCHANGE        b3lyp
   BASIS           cc-pvdz
$end

@@@

$molecule
   read
$end

$rem
   BASIS           cc-pvdz
   METHOD          ccsd
   SCF_GUESS       read
   MAX_SCF_CYCLES  0
   N_FROZEN_CORE   1
$end