MOLPRO

47 POTENTIAL ENERGY SURFACES (SURF) 355
are termed multi-level calculations. There does not exist a corresponding set of keywords for
the 4-body terms. 4-body terms will always use the variables specified for the 3-body terms.
MULTI=value
The keywords START1D, START2D, START3D in combination with
the commands VAR1D, VAR2D and VAR3D allow for the calculation
of multi-level potential energy surfaces. This would imply in principle
that the 1D term of the potential needs to be computed at all three
levels and the 2D term at two computational levels. As certain low
level results are a byproduct of more sophisticated methods (e.g. the
HF energy is a byproduct of a MP2 calculation or the MP2 energy is a
byproduct of a CCSD(T) calculation) the computational overhead can
be avoided by the MULTI option.
MULTI=1: This is the default and most expensive choice. The 1D
potential will be computed at all 3 levels of theory. Likewise, the 2D
potential will be calculated at 2 levels explicitly. An example would
be:
1D: CCSD(T)/cc-pVTZ
2D: MP4(SDQ)/cc-pVTZ
3D: MP2/cc-pVDZ
{SURF,Start1D=label1
VMULT,Start2D=label2,Start3D=label3,Multi=1}
MULTI=2: All information is provided by the preceding calculations
and thus no part of the potential has to be computed twice. Examples:
1D: CCSD(T)/cc-pVTZ
2D: CCSD(T)/cc-pVTZ
3D: MP2/cc-pVTZ
{SURF,Start1D=label1
VMULT,Start2D=label1,Start3D=label2
VMULT,Var3D=EMP2,Multi=2}
1D: CCSD(T)/cc-pVTZ
2D: MP2/cc-pVTZ
3D: MP2/cc-pVTZ
{SURF,Start1D=label1
VMULT,Start2D=label2,Start3D=label2
VMULT,Var2D=EMP2,Var3D=EMP2,Multi=2}
MULTI=3: The 2D potential provides all information for the 3D part
while there is no connection between 1D and 2D. Consequently, The
1D contributions need to be computed twice (at the 1D and 2D levels)
while all other terms will be computed just once. Examples:
1D: CCSD(T)/cc-pVTZ
2D: MP4(SDQ)/cc-pVTZ
3D: MP2/cc-pVTZ
{SURF,Start1D=label1
VMULT,Start2D=label2,Start3D=label3
VMULT,Var3D=EMP2,Multi=3}