Multi-reference electron correlation with large active spaces from time-dependent perturbation theory with density matrix renormalization group

In the recent years there has been a remarkable progress in the development of methods for the description of strong electron correlation in mols. There are now many methods that describe strong correlation in active spaces with up to 60 orbitals. However, capturing the remaining weak (dynamic) correlation outside of the active space remains a challenge. Conventional multi-ref. methods for dynamic correlation (such as internally-contracted perturbation theory, e.g. CASPT2 or NEVPT2) require computation of up to fourth-order reduced d. matrixes, which prohibits their application to systems with active spaces with more than 24 orbitals. In this talk I will describe a new method which provides an efficient description of static and dynamic electron correlation by combining the formalism of d. matrix renormalization group (DMRG) and timedependent multi-ref. perturbation theory. This approach has a no. of important advantages:. 1) It is equiv. to fully uncontracted perturbation theory, but has a lower computational scaling than the contracted approaches;. 2) It avoids computation of the three- and four-particle reduced d. matrixes;. 3) It has a polynomial scaling with the no. of active orbitals and can be applied to active spaces with much more than 24 orbitals. In my talk, I will present a brief overview of multi-ref. time-dependent perturbation theory, discuss its implementation in combination with DMRG, and demonstrate its performance for strongly correlated systems with large active spaces