Q-Chem 5.1 User’s Manual

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.[Beran et al.(2003)Beran, Gwaltney, and Head-Gordon] This can be achieved by first doing DFT calculation and then reading the orbitals and turning the Hartree-Fock procedure off. A more convenient way is just to specify EXCHANGE, e.g., EXCHANGE = B3LYP means that B3LYP orbitals will be computed and used in the CCMAN/CCMAN2 module, as in the following example.

Example 6.95  CCSD calculation of triplet methylene using B3LYP orbitals

   0 3
   H  1  CH
   H  1  CH  2  HCH

   CH  = 1.07
   HCH = 111.0

   JOBTYPE         sp      single point
   EXCHANGE        b3lyp
   CORRELATION     ccsd
   BASIS           cc-pvdz