Editing COMPASS force field

'''COMPASS''' ('''C'''ondensed-phase '''O'''ptimized '''M'''olecular '''P'''otentials for '''A'''tomistic '''S'''imulation '''S'''tudies) is a member of the consistent family of [[force fields]] ([[CFF91]], [[PCFF]], [[CFF]] and COMPASS), which are closely related second-generation force fields. They were parameterized against a wide range of experimental observables for organic compounds containing [[hydrogen |H]], [[carbon |C]], [[nitrogen |N]], [[oxygen |O]], [[Sulfur |S]], [[Phosphorus |P]], halogen atoms and ions, alkali metal cations, and several biochemically important divalent metal cations. PCFF is based on CFF91, extended so as to have a broad coverage of organic polymers, (inorganic) metals, and [[zeolites]]. COMPASS is the first force field that has been parameterized and validated using condensed phase properties in addition to various  and empirical data for molecules in isolation. Consequently, this forcefield enables accurate and simultaneous prediction of structural, conformational, vibrational, and thermo-physical properties for a broad range of molecules in isolation and in condensed phases.
==Functional form==
The COMPASS force field (Ref. 1 Eq. 1) consists of terms for bonds (<math>b</math>), angles (<math>\theta</math>), dihedrals (<math>\phi</math>), out-of-plane angles (<math>\chi</math>) as well as cross-terms, and two non-bonded functions, a  [[Coulomb's law |Coulombic function]] for electrostatic interactions and a [[9-6 Lennard-Jones potential]] for van der Waals interactions.

:<math>E_{\mathrm{total}} = E_b + E_{\theta} + E_{\phi} + E_{\chi} + E_{b,b'} + E_{b,\theta} + E_{b,\phi} +  E_{\theta,\phi} + 
E_{\theta,\theta'} +  E_{\theta,\theta',\phi} + E_{q} + E_{\mathrm{vdW}}</math>

where

:<math>E_b = \sum_b \left[ k_2 (b-b_0)^2 + k_3 (b-b_0)^3 + k_4 (b-b_0)^4   \right]</math>

:<math>E_{\theta} = \sum_{\theta} \left[ k_2 (\theta-\theta_0)^2 + k_3 (\theta-\theta_0)^3 + k_4 (\theta-\theta_0)^4   \right]</math>

:<math>E_{\phi} = \sum_{\phi} \left[ k_1(1-\cos \phi) + k_2(1-\cos 2\phi) + k_3(1-\cos 3\phi) \right]</math>

:<math>E_{\chi} = \sum_{\chi} k_2 \chi^2</math>

:<math>E_{b,b'} = \sum_{b,b'} k(b-b_0)(b'-b_0')</math>

:<math>E_{b,\theta} = \sum_{b,\theta} k(b-b_0)(\theta-\theta_0)</math>

:<math>E_{b,\phi} = \sum_{b,\phi} (b-b_0) \left[k_1 cos \phi + k_2 cos 2\phi + k_3 cos 3\phi\right]</math>

:<math>E_{\theta,\phi} = \sum_{\theta,\phi} (\theta-\theta_0) \left[k_1 cos \phi + k_2 cos 2\phi + k_3 cos 3\phi\right]</math>

:<math>E_{\theta,\theta'} = \sum_{\theta,\theta'} k(\theta-\theta_0)(\theta'-\theta_0')</math>

:<math>E_{\theta,\theta',\phi} = \sum_{\theta,\theta',\phi} k(\theta-\theta_0)(\theta'-\theta_0')\cos \phi</math>

:<math>E_{q}=\sum_{ij} \frac{q_i q_j}{r_{ij}}</math>

and

:<math>E_{\mathrm{vdW}}= \sum_{ij} \epsilon_{ij} \left[ 2 \left(\frac{r_{ij}^0}{r_{ij}} \right)^9 -3  \left(\frac{r_{ij}^0}{r_{ij}} \right)^6\right]</math>

with the  [[combining rules#Waldman-Hagler rules |Waldman-Hagler combining rules ]] <ref>[http://dx.doi.org/10.1002/jcc.540140909 M. Waldman and A. T. Hagler "New combining rules for rare-gas Van der-Waals parameters", Journal of Computational Chemistry '''14''' pp.  1077-1084 (1993)]</ref> (Note, in Ref. 1 Eq. 3 seems to have a typographical error):

:<math>r_{ij}^0 = \left( \frac{ (r_i^0)^6 + (r_j^0)^6 }{2} \right)^{1/6}</math>

and

:<math>\epsilon_{ij} = 2 \sqrt{\epsilon_i \cdot \epsilon_j} \left( \frac{ (r_i^0)^3 \cdot  (r_j^0)^3 }{ (r_i^0)^6  + (r_j^0)^6 }  \right)</math>

==Parameters==
==References==
#[http://dx.doi.org/10.1021/jp980939v H. Sun "COMPASS:  An ab Initio Force-Field Optimized for Condensed-Phase Applications - Overview with Details on Alkane and Benzene Compounds", Journal of Physical Chemistry B '''102''' pp. 7338–7364 (1998)]
#[http://dx.doi.org/10.1016/S1089-3156(98)00042-7  H. Sun, P. Ren and J. R. Fried "The COMPASS force field: parameterization and validation for phosphazenes", Computational and Theoretical Polymer Science '''8''' pp. 229-246 (1998)] 
#[http://dx.doi.org/10.1021/jp991786u S. W. Bunte and H. Sun "Molecular Modeling of Energetic Materials:  The Parameterization and Validation of Nitrate Esters in the COMPASS Force Field", Journal of Physical Chemistry B '''104''' pp. 2477-2489 (2000)]
#[http://dx.doi.org/10.1021/jp992913p Jie Yang, Yi Ren, An-min Tian and Huai Sun "COMPASS Force Field for 14 Inorganic Molecules, He, Ne, Ar, Kr, Xe, H2, O2, N2, NO, CO, CO2, NO2, CS2, and SO2, in Liquid Phases", Journal of Physical Chemistry B '''104''' pp. 4951-4957 (2000)]
#[http://dx.doi.org/10.1002/jcc.10316 Michael J. McQuaid, Huai Sun and David Rigby "Development and validation of COMPASS force field parameters for molecules with aliphatic azide chains", Journal of Computational Chemistry '''25''' pp. 61-71 (2004)]
[[category: force fields]]