This is a general method for treating two variants of the multireference perturbation theory

  1. multi-reference linearized coupled cluster (MRLCC)

  2. N-electron valence perturbation theory (NEVPT2)

Both these methods are accurate and don't have the usual intruder-state problems that plague several multireference perturbation theories such as CASPT2. The unique feature of this program is that the first-order wavefunction is represented with a combination of internal contraction (IC) and matrix product state (MPS), which allows us to perform calculations without ever needing to calculate or approximate the fourth order reduced density matrix (RDM). Our code is currently interfaced with the PyScf code developed by Qiming Sun and is distributed under the GNU  public license. To obtain a copy please contact me by email


The table shows the wall clock time (in units of 1000 seconds) required to perform the various calculations on a single node containing two Intel Xeon E5-2680 v2 processors of 2.80 GHz each and 128 giga-byte memory.