Sungkwang Mun
Co-authors: Ric Carino and Michael Baskes
Center for Advanced Vehicular Systems (CAVS)
Mississippi State University

MEAM with Bond Order force field Implementation in LAMMPS

Modified Embedded Atom Method (MEAM) force field that been extended to include the bond order to describe the energetics of unsaturated hydrocarbons. Two types of unsaturated bonds (double and triple) energies are added to the original MEAM equation. The new formalism is called Modified Embedded Atom Method with Bond Order (MEAM-BO) is a step towards developing more reliable and accurate polymer simulations with their associated structure-property relationships, such as reactive multicomponent (organic/metal) systems, polymer-metal interfaces, and nanocomposites.

Also, new bond order parameters as well as updated original MEAM parameters for hydrocarbons give improved results over the previous work. Such quantities like bond lengths, bond angles, and atomization energies at 0 K, dimer molecule interactions, rotational barriers, and the pressure-volume-temperature relationships of dense systems of small molecules give a comparable or more accurate property relative to experimental and first-principles data than the classical reactive force fields REBO and ReaxFF.

We implemented the additional bond order equations in the LAMMPS code by modifying the existing C++ version of the MEAM code in LAMMPS. Specifically, the number of possible bonds is determined by examining the pairs of atoms in the system based on the screening values of the original MEAM. If there is any bond pair, local density terms for bond order are then calculated and exchanged across the MPI subdomains followed by the determination of the bond type/order and the final calculation of the energy (and force) with respect to bond order. When if there is no unsaturated bond or the energies with respect to bond order are zero, MEAM-BO reduces to the original MEAM force field. In order words, if any atom pair does now show the bond order character, the results will be the same as those of the original MEAM. As such, MEAM-BO force field can describe the interaction of organic materials with metals within the same MEAM formalism. We found that the calculation speed of MEAM-BO is about 2 times slower than the original MEAM in case of 12,000 atoms of bulk benzene system in serial calculation. This result is based on the speed comparison using our in-house code (DYNAMO) not LAMMPS. The current goal is to achieve the similar performance in LAMMPS.