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# 11.5.1 Introduction

(December 20, 2021)

The Effective Fragment Potential (EFP) method is a computationally inexpensive way of modeling intermolecular interactions in non-covalently bound systems. The EFP approach can be viewed as a QM/MM scheme with no empirical parameters. Originally EFP was developed by Prof. Mark Gordon’s group, 255 Day P. N. et al.
J. Chem. Phys.
(1996), 105, pp. 1968.
, 388 Gordon M. S. et al.
J. Phys. Chem. A
(2001), 105, pp. 293.
and was implemented in gamess. 1003 Schmidt M. W. et al.
J. Comput. Chem.
(1983), 14, pp. 1347.
A review of the EFP theory and applications can be found in Ref.  357 Ghosh D. et al.
J. Phys. Chem. A
(2010), 114, pp. 12739.
, 387 Gordon M. S. et al.
Chem. Rev.
(2012), 112, pp. 632.
. A related approach, also based on distributed multipoles, is called XPol; it is described in Section 12.12.

A new implementation of the EFP method based on the libefp library by Dr. Ilya Kaliman (see https://libefp.github.io) has been added to Q-Chem. 544 Kaliman I. A., Slipchenko L. V.
J. Comput. Chem.
(2013), 34, pp. 2284.
, 545 Kaliman I. A., Slipchenko L. V.
J. Comput. Chem.
(2015), 36, pp. 129.
The new EFP module is called EFPMAN2. EFPMAN2 can run calculations in parallel on shared memory multi-core computers and clusters of computers. EFPMAN2 is interfaced with the CCMAN and CCMAN2 modules to allow coupled cluster and EOM-CC calculations with EFP. CIS and TDDFT calculations with EFP are also available.