6.10 Coupled Cluster Active Space Methods

6.10.2 VOD and VOD(2) Methods

The VOD method is the active space version of the OD method described earlier in Section 6.8.3. Both energies and gradients are available for VOD, so structure optimization is possible. There are a few important comments to make about the usefulness of VOD. First, it is a method that is capable of accurately treating problems that fundamentally involve 2 active electrons in a given local region of the molecule. It is therefore a good alternative for describing single bond-breaking, or torsion around a double bond, or some classes of diradicals. However it often performs poorly for problems where there is more than one bond being broken in a local region, with the non variational solutions being quite possible. For such problems the newer VQCCD method is substantially more reliable.

Assuming that VOD is a valid zero order description for the electronic structure, then a second-order correction, VOD(2), is available for energies only. VOD(2) is a version of OD(2) generalized to valence active spaces. It permits more accurate calculations of relative energies by accounting for dynamical correlation.

Example 6.24  Calculate the correlation energy of the water molecule with partially stretched bonds, the VOD coupled-cluster active space method. This is a relatively “easy” job to converge, and may be contrasted with the next example, which is not easy to converge. The orbitals are restricted.

$molecule
  0  1
  O
  H  1  r
  H  1  r  2  a

  r =   1.5
  a = 104.5
$end

$rem
   METHOD        vod
   BASIS         6-31G
$end

Example 6.25  The water molecule with highly stretched bonds, calculated via the VOD coupled-cluster active space method. This is a “difficult” job to converge. The convergence options shown permitted the job to converge after some experimentation (thanks due to Ed Byrd for this!). The difficulty of converging this job should be contrasted with the previous example where the bonds were less stretched.

$molecule
   0  1
   O
   H  1  r
   H  1  r  a

   r =   3.0
   a = 104.5
$end

$rem
   METHOD                vod
   BASIS                 6-31G
   SCF_CONVERGENCE       9
   THRESH                12
   CC_PRECONV_T2Z        50
   CC_PRECONV_T2Z_EACH   50
   CC_DOV_THRESH         7500
   CC_THETA_STEPSIZE     3200
   CC_DIIS_START         75
$end