6 Wave Function-Based Correlation Methods

6.12 Non-Hartree-Fock Orbitals in Correlated Calculations

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.80 This can be achieved by first doing DFT calculation and then reading the orbitals and turning the SCF procedure off.

Example 6.28  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