Apply this tag to questions about methods that use multiple determinants to construct the wave function, e.g. MCSCF, MR-PT, CASSCF, MR-CI, etc., questions about chemical systems that might require these methods.
Multi-reference electron correlation methods are necessary to treat strongly correlated electrons. This kind of strong correlation is also often referred to as static correlation.
These methods usually employ and optimise multiple determinants in their ansatz of the wave function.
Examples for these methods include multi-configurational self-consistent field (MCSCF), multi-reference perturbation theory (MR-PT), complete active space (CAS) methods, multi-reference configuration interaction (MR-CI), multi-reference coupled cluster (MR-CC), and more. An introduction into the topic can be found at Wikipedia.
This tag should be applied (in conjunction with theoretical-chemistry) to questions about these methods; it should also be applied to questions about chemical systems that may require these methods (possibly in conjunction with computational-chemistry).