32.2 First example

A typical input for SAPT has the following form:

r=5.6
geometry={nosym; he1; he2,he1,r}
basis=avqz

!wf records
ca=2101.2
cb=2102.2

!monomer A
dummy,he2
{hf; save,$ca}
sapt;monomerA

!monomer B
dummy,he1
{hf; start,atdens; save,$cb}
sapt;monomerB

!interaction contributions
sapt;intermol,ca=$ca,cb=$cb

Here the sapt;monomerA/B store some informations about the two monomers which are needed in the subsequent SAPT calculation invoked by sapt;intermol. The individual interaction energy terms are stored (in millihartree) in distinct variables and may be collected in arrays for producing potential energy surfaces. For example the input

geometry={nosym; he1; he2,he1,r}
basis=avtz

!wf records
ca=2101.2
cb=2102.2

!distances
dist=[4.5,5.0,5.5,5.6,6.0,6.5,7.0]

do i=1,#dist
   r=dist(i)
   
   !monomer A
   dummy,he2
   {hf; save,$ca}
   sapt;monomerA

   !monomer B
   dummy,he1
   {hf; start,atdens; save,$cb}
   sapt;monomerB

   !interaction contributions
   sapt;intermol,ca=$ca,cb=$cb

   elst(i)=E1pol;  exch(i)=E1ex
   ind(i)=E2ind;   exind(i)=E2exind
   disp(i)=E2disp; exdisp(i)=E2exdisp
   etot(i)=E12tot

   data,truncate,$ca
enddo

{table,dist,elst,exch,ind,exind,disp,exdisp,etot
ftyp,d,d,d,d,d,d,d,d,d
plot}

yields the plot

Currently SAPT only accepts single-determinant wave functions for the monomers, i.e. from Hartree-Fock or Kohn-Sham DFT (see next section) calculations. This means that if Hartree-Fock wave functions are used for monomer, the following quantity is obtained (zero in superscript denotes that no intramonomer correlation is accounted for) [1].

$$\begin{eqnarray*} E_\mathrm{SAPT}=E^{(10)}_\mathrm{pol}+E^{(10)}_\mathrm{exch}+ ... ...-ind,resp.}+E^{(20)}_\mathrm{disp}+ E^{(20)}_\mathrm{exch-disp} \end{eqnarray*}$$

No point group symmetry can be exploited in a SAPT calculation.