This package contains a LAMMPS implementation of the electron Force Field (eFF) currently under development at Caltech, as described in A. Jaramillo-Botero, J. Su, Q. An, and W.A. Goddard III, JCC,

2010. The eFF potential was first introduced by Su and Goddard, in 2007.

eFF can be viewed as an approximation to QM wave packet dynamics and Fermionic molecular dynamics, combining the ability of electronic structure methods to describe atomic structure, bonding, and chemistry in materials, and of plasma methods to describe nonequilibrium dynamics of large systems with a large number of highly excited electrons. We classify it as a mixed QM-classical approach rather than a conventional force field method, which introduces QM-based terms (a spin-dependent repulsion term to account for the Pauli exclusion principle and the electron wavefunction kinetic energy associated with the Heisenberg principle) that reduce, along with classical electrostatic terms between nuclei and electrons, to the sum of a set of effective pairwise potentials. This makes eFF uniquely suited to simulate materials over a wide range of temperatures and pressures where electronically excited and ionized states of matter can occur and coexist.

The necessary customizations to the LAMMPS core are in place to enable the correct handling of explicit electron properties during minimization and dynamics.

See the doc page for the pair_style eff/cut command to get started.

There are example scripts for using this package in examples/USER/eff.

There are auxiliary tools for using this package in tools/eff.

The person who created this package is Andres Jaramillo-Botero at CalTech (ajaramil at wag.caltech.edu). Contact him directly if you have questions.

---

AUTHOR INFORMATION:

Andres Jaramillo-Botero California Institute of Technology (Caltech) Chemistry and Chemical Engineering, 139-74 1200 E. California Blvd., Pasadena, CA 91125 Phone: (626) 395-3591 e-mail: ajaramil@wag.caltech.edu

Co-Authors: Julius Su (jsu@wag.caltech.edu) William A. Goddard III (wag@wag.caltech.edu)

ACKNOWLEDGMENTS:

Thanks to Steve Plimpton and Aidan Thompson for their input on the LAMMPS architecture and for their help in customizing some of the required LAMMPS core modules.

Version 01/2010: Special thanks to:

• Hai Xiao (Caltech) for reviewing the fixed-core implementation and providing useful insights to improve it, and for his work on the effective core pseudopotential.
• Vaclav Cvicek (Caltech) for thoroughly revising the units, for finding a bug in the fix_langevin_eff radial scaling factors, and for suggesting changes to clean-up the code.
• Patrick Theofanis (Caltech) for providing an optimized set of parameters for the Si ECP (default) and for providing basic cases.
• Qi An (Caltech) for providing feedback on usage, application cases, and testing.

VERSION NOTES:

01/2010: Added support for fixed-core and effective core pseudopotentials [ECP] (useful for C, Al, Si, O and other elements). Cleaned up the code to make it easier to maintain, revised support for real units, upgraded post-processing and visualization tools, added support for "compute pair eff" to allow thermo prints with the different eFF energy components (eke, epauli, ecoul and errestrain), fixed radial scaling factors in the eff langevin thermostat.

12/2011: Added support for "zero" option in fix langevin/eff (see doc), and adjusted fix_langevin_eff.cpp to correctly thermostat between nuclear and electronic dof (required additional scaling of friction term in the Langevin equations of motion). Radial electron mass now scales as a function of system dimension.

Bug fixes: (10-2011): Thanks to Christian Chenard-Lemire (U Montreal) for reporting a bug in the fixed pair_eff_cut.cpp fixedcore-pseudocore interactions (an incorrect index and a missing elec-core call to account for the 2 electrons from the fixed core.) (12-2011): Corrected undefined natom variable in fix_langevin_eff (recent changes in main fix_langevin class caused compilation error in user-eff). Corrected thermostat in fix langevin/eff as described in version 12/2011.