# Perturbation theory. A reference on computational aspects

*From*: boufer <boufer { *at * } cennas.nhmfl.gov>
*Organization*: None
*Subject*: Perturbation theory. A reference on computational
aspects
*Date*: Mon, 24 Aug 1998 10:26:45 -0400

Greetings all,
I wonder if anybody coud suggest a good reference regarding the
computational aspects of perturbation theory. As a matter I am a little
confused as regard the application of such a procedure using the results
of a Hartree-Fock calculation.
Let me get more explicit :
Let's assume for simplicity that one has a one-electron perturbation
H' = Sum_i h(i). Then, if my knowledge in this perturbation "business"
is right, the function~(first order correction) Psi' is expanded as a
linear combination of the determinants corresponding to the ground
state, first "excited" determinant, second "excited"
determinanant, ...
This is the RSPT.
As a consequence of this, the second order correction of the energy
should involve only the first "excited" determinant~(for a two
particle
pertubation as in MP2 for instance we will use only the second
"excited"
determinant.)
We know that the above expansion yields an exact answer if the basis set
is complete.
The question is : Is there any procedure allowing us to choose the basis
functions in order to get accurate results though the size of the basis
is finite ?
Thanks for your time and Help.
--
Ahmed