The Paesani Research Group

Laboratory for Theoretical and Computational Chemistry at UC San Diego  

MB-pol in OpenMM

(version 05.2016)

mbpol is a plugin publicly available on Github for the OpenMM toolkit for molecular simulations using the MB-pol, our many-body water potential [1-3].

Currently, the mbpol plugin is available in the “reference implementation” of OpenMM, which is appropriate for energy calculations for both clusters and systems in periodic boundary conditions and short simulations. Multicore and GPU implementations for long molecular dynamics production runs will become available in the near future.

Some notes about the MB-pol many-body potential for water

The development of MB-pol started with a detailed analysis of the two- and three-body water interactions evaluated at the CCSD(T) level to quantitatively assess the accuracy of current force fields, DFT models, and ab initio based interaction potentials that are commonly used in molecular simulations [1]. On the basis of this analysis and the results obtained with the HBB2-pol potential [2], the full-dimensional MB-pol potential was developed entirely from "first principles" building upon the many-body expansion of the interaction energy of water molecules [3, 4, 5].

MB-pol explicitly treats the one-body (intramolecular distortion energy) term and the short-ranged two- and three-body terms. MB-pol can thus be viewed as a classical polarizable potential supplemented by short-range two- and three-body terms that effectively represent quantum-mechanical interactions arising from the overlap of the monomer electron densities. Specifically, at all separations, the total MB-pol two-body term includes (damped) dispersion forces derived from ab initio computed asymptotic expansions of the dispersion energy along with electrostatic contributions due to the interactions between the molecular permanent and induced moments. At short-range, this two-body term is supplemented by a 4th-degree permutationally invariant polynomial that smoothly switches to zero as the oxygen-oxygen separation in the dimer approaches 6.5 Å. Similarly, the MB-pol three-body term includes a three-body polarization term at all separations, which is supplemented by a short-range 4th-degree permutationally invariant polynomial that effectively corrects for the deficiencies of a purely classical representation of the three-body interactions in regions where the electron densities of the three monomers overlap. This short-range three-body contribution is smoothly switched off once the oxygen-oxygen separation between any water molecule and the other two water molecules of a trimer reaches a value of 4.5 Å. In MB-pol, all induced interactions are described through many-body polarization. MB-pol thus contains many-body effects at all monomer separations as well as at all orders, in an explicit way up to the third order and in a mean-field fashion at all higher orders.

Without containing any empirical parameters, MB-pol accurately describes the properties of gas-phase clusters, including the dimer vibration-rotation tunneling spectrum [3], the second and third virial coefficients [3, 4], cluster structures and energies [4]. Simulations carried out with path-integral molecular dynamics (PIMD) and centroid molecular dynamics (CMD) demonstrate that MB-pol provides a highly accurate description of the liquid phase of water at ambient conditions in comparison with experiment for several structural, thermodynamic, and dynamical properties [5].

Finally, many-body molecular dynamics (MB-MD) simulations carried out with MB-pol in combination with many-body representations of the dipole moment and polarizability predict infrared (IR) and Raman spectra of liquid water [6] and the vibrational sum-frequency generation spetrum of the air/water interface [7] in excellent agreement with the experimental results.

Based on a comparison with quantum Monte Carlo reference energies for liquid water it has also been shown that MB-pol achieves higher accuracy than existing models based on density functional theory that are commonly used in ab initio molecular dynamics simulations of liquid water [8].


1) G.R. Medders, V. Babin, F. Paesani, A critical assessment of two-body and three-body interactions in water, J.

    Chem. Theory Comput. 9, 1103 (2013).

2) V. Babin, G.R. Medders, F. Paesani, Toward a universal water model: First principles simulations from the dimer

    to the liquid phase, J. Phys. Chem. Lett. 3, 3765 (2012).

3) V. Babin, C. Leforestier, F. Paesani, Development of a “first principles" water potential with flexible monomers:

    Dimer potential energy surface, VRT spectrum, and second virial coefficient, J. Chem. Theory Comput. 9, 5395,


4) V. Babin, G.R. Medders, F. Paesani, Development of a “first principles" water potential with flexible monomers.

    II: Trimer potential energy surface, third virial coefficient, and small clusters, J. Chem. Theory Comput. 10, 1599


5) G.R. Medders, V. Babin, F. Paesani, Development of a “first principles" water potential with flexible monomers.

    III: Liquid phase properties, J. Chem. Theory Comput. 10, 2906 (2014).

6) G.R. Medders, F. Paesani, Infrared and Raman spectroscopy of liquid water through “first principles" many-body

    molecular dynamics, J. Chem. Theory Comput. 11, 1145 (2015).

7) G.R. Medders, F. Paesani, Dissecting the molecular structure of the air/water interface from quantum simulations

    of the sum-frequency generation spectrum, J. Am. Chem. Soc. 138, 3912 (2016).  

8) G.R. Medders, A.W. Götz, M.A. Morales, F. Paesani, On the representation of many-body interactions in

    water, J. Chem. Phys. 143, 104102 (2015).

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