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(June 30, 2021)

The VOD method is the active space version of the OD method described earlier in Section 6.10.4. 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.

$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

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