Posted by Steven Vancoillie on March 26, 2015 at 08:53:03:

In Reply to: CASPT2 iterations posted by Roser on March 10, 2015 at 10:07:03:

: I was doing CASPT2 calculations and I saw that an iterative procedure is needed, but I don't understand why. CASPT2 method does not need iterations.

: I have read in the manual (section 10.5 Excited states/CASPT2): "Of course, the complete Hˆ0 is used to solve the CASPT2 equations, which is why an iterative procedure is needed."

The CASPT2 equation is solved by means of a preconditioned conjugate gradient method, which is an iterative method to solve Ax = b.

: I had previously problems on the convergence and I would like to understand these iterations.

Problems in convergence usually point to some problematic CAS calculation. You can explain your problem in more detail or post your input/output to get more advice on how to proceed.

: Thank you!

: Roser