Accurate quantum computational chemistry has evolved dramatically. The size of molecular systems, which can be studied accurately using molecular theory is increasing very rapidly. Theoretical chemistry has opened up a world of new possibilities. It can treat real systems with predictable accuracy. Computational chemistry is becoming an integral part of chemistry research. Theory can now make very signicant contribution to chemistry.

This review will focus on our recent developments in the theoretical and computational methodology for the study of molecular structure and molecular interactions. We are aiming at developing accurate molecular theory on systems containing hundreds of atoms. We continue our research in the following three directions: (i) development of new ab initio theory, particularly multireference-based perturbation theory, (ii) development of exchange and correlation functionals in density functional theory, and (iii) development of molecular theory including relativistic eects.

We have enjoyed good progress in each of the above areas. We are very excited about our discoveries of new theory and new algorithms and would like to share this enthusiasm with readers.

Keywords: Correlation effects; DFT; relativistic effects; MRMP; MC-QDPT; RESC; DK3; OP and parameter-free functionals