Variational bounds and their limits
Research context
postgraduate concept · established · As of 2026-10-10
For a normalized trial state of the same Hermitian, lower-bounded Hamiltonian, the Rayleigh energy cannot be below its ground-state energy.
Key knowledge
For a normalized trial state of the same Hermitian, lower-bounded Hamiltonian, the Rayleigh energy cannot be below its ground-state energy. Truncated variational CI retains this bound; ordinary coupled-cluster and perturbation energies do not inherit it.
Values and equations
E[psi] = <psi|H|psi>/<psi|psi> >= E0
- E[psi]
- Rayleigh energy J
- psi
- admissible nonzero trial wavefunction
- H
- same electronic Hamiltonian
- E0
- exact ground energy of that Hamiltonian J
Conditions and scope
- The Hamiltonian, electron number and domain must be held fixed; an energy bound is not a bound on every molecular property.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Configuration Interaction Theory: Variational theorem (pp. 17–20); size consistency/extensivity (pp. 31–33) (extracted lecture-note sections)
2026-10-10 · Bounded author source inspection of variational bounds and their limits, including the stated conditions; independent branch review is recorded separately.
Entry details
Permanent link to this entry ↗- Record identity
- res-chem-variational-bound
- Revision
- research.1
- Structured reference record
- Postgraduate & research reference