  The Density Matrix Renormalisation Group (DMRG) is an electronic structure
method that has recently been applied to ab-initio quantum chemistry. Even at
this early stage, it has enabled the solution of many problems that would
previously have been intractable with any other method, in particular,
multireference problems with very large active spaces. Historically, the DMRG
was not originally formulated from a wavefunction perspective, but rather in a
Renormalisation Group (RG) language. However, it is now realised that a
wavefunction view of the DMRG provides a more convenient, and in some cases
more powerful, paradigm. Here we provide an expository introduction to the DMRG
ansatz in the context of quantum chemistry.
