MBPT vs CI

 

• MBPT is not variational. Each level of MBPT is not an upper bound to the exact energy (i.e. lower energy not necessarily better).
• CI methods are variational. Predicted energies provide an upper bound to the exact energy.
• MBPT is size-extensive and size-consistent.
• Truncated CI methods are neither size-extensive nor size-consistent.
• Full-CI is variational, size-extensive and size-consistent.

For many quantum chemical applications, it is much more important that a method be size-consistent as opposed to being variational. For an excellent discussion of size-consistency and size-extensivity please refer to Section 2.4 of Ref [5]. Section 4.6 of Ref [4] also contains information regarding size-consistency.