1 Introduction

Mrcc is a suite of ab initio and density functional quantum chemistry programs for high-accuracy electronic structure calculations developed and maintained by the quantum chemistry research group at the Department of Physical Chemistry and Materials Science, TU Budapest [66]. Its special feature, the use of automated programming tools enabled the development of tensor manipulation routines which are independent of the number of indices of the corresponding tensors, thus significantly simplifying the general implementation of quantum chemical methods. Applying the automated tools of the program several quantum chemistry models and techniques of high complexity have been implemented so far including arbitrary single-reference coupled-cluster (CC) and configuration interaction (CI) methods, multi-reference CC approaches, CC and CI energy derivatives and response functions, arbitrary perturbative CC approaches. Many features of the package are also available with relativistic Hamiltonians allowing for accurate calculations on heavy element systems. The developed cost-reduction techniques and local correlation approaches also enable high-precision calculations for medium-sized and large molecules.