[molpro-user] Computation of the dipole moment with CCSD(T)

[Lorenzo Lodi]

I am computing the dipole moment of water with various methods and I'd 
now like to use the CCSD(T) method.

At the CCSD level I can compute the dipole with
---
RHF
CCSD ; CORE, 0,0,0,0
EXPEC, DM
---
and I understand that the value given as "orbitally relaxed CCSD dipole 
moment" is the same value as the expectation value of \mu on the CCSD 
wavefunction. (Incidentally, the GEXPEC directive does not work for me, 
is this behaviour normal?)

Now, for the CCSD(T) dipole.
Looking at some old posts it was suggested to calculate it using the DIP 
keyword with a small field and then taking the derivative of the energy 
at zero field. Now, my objection is as follows: the Hellmann-Feynmann 
theorem, on which this finite-field approach is based, holds for
1) the exact wavefunction
and
2) a wavefunction which is variationally optimised in all its parameters
The coupled cluster method is, of course, non-variational so I don't 
expect the finite-field approach to work well in this case.
In fact, I verified this comparing the CCSD dipole obtained by 
finite-field and as given by molpro (equilibrium geometry, cc-pV6Z 
basis) and I got a difference of ~0.003 a.u., which is too large for the 
level of accuracy I am looking for.

My conclusion would be that there is no accurate way to compute the 
dipole with the CCSD(T) method (in fact, as there is no CCSD(T) 
wavefunction, this may not be anything new...).

Could anyone confirm these comments and/or give any suggestion about how 
to proceed in this direction?

Thank you.

Lorenzo Lodi