. Assume H is dividable into a solvable part: 
with 
and 
Perturbation theory is used for problems which are too difficult.
. Assume H is dividable into a solvable part: with and
Example: Helium 
The last term is 
, the other terms are just the
solution for the hydrogen twice in a tensor product. Solution of 
,
. Then 
.
In matrix language: so 
. Unfortunately, this is not easily solvable.
Method: start with one 
and see how it changes as is turned on.
for 
very small.
Theory developed in perturbation expansion.