# Welcome to BLOCK’s documentation!¶

BLOCK implements the density matrix renormalization group (DMRG) algorithm for quantum chemistry. The DMRG is a variational wavefunction method. Compared to other quantum chemical methods, it efficiently describes strong, multi-reference correlation in a large number of active orbitals (occupancies far from 0 or 2). The method is also provably optimal for correlation with a one-dimensional topology, that is, where orbitals are arranged with a chain- or ring-like connectivity. However, with the possible exception of small molecules, for correlation that is dynamic in character, the DMRG may be less computationally efficient than other methods such as coupled cluster theory or multireference configuration interaction. We recommend the use of the DMRG in problems requiring active spaces too large for standard complete active space (CAS) techniques. Thus, if you are interested in:

- a CAS-like treatment of low-lying eigenstates in real problems with more than 50 active orbitals,
- or, one-dimensional orbital topologies with up to 100 active orbitals,
- and, standard chemical accuracy (1 kcal/mol in energy differences),

then the DMRG may be the right method for you.

## Contents¶

- Overview
- Block Installation
- DMRG for electronic structure calculations
- Block program as a standalone solver
- Molecular symmetry
- State wavefunction
- State-averaged calculation
- State-specific calculation
*n*-particle reduced density matrix- 1- and 2-particle transition reduced density matrix
- Restart DMRG energy calculation
- Restart DMRG
*n*-particle reduced density matrix calculation - Restart DMRG transition reduced density matrix calculation
- Customize sweep schedule
- Sweep energy extrapolation
- Further Reading

- Keywords
- Benchmark
- Change Log