forcefield_template.xml

<ForceField>
 <AtomTypes>
  <Type name="opls_135" class="CT" element="C" mass="12.01100" def="[C;X4](H)(H)(H)C" desc="alkane CH3"/>
  <Type name="opls_136" class="CT" element="C" mass="12.01100" def="O" desc="alkane CH2"/>
  <Type name="opls_137" class="CT" element="C" mass="12.01100" def="O" desc="alkane CH"/>
  <Type name="opls_138" class="CT" element="C" mass="12.01100" def="O" desc="alkane CH4"/>
  <Type name="opls_139" class="CT" element="C" mass="12.01100" def="O" desc="alkane C"/>
  <Type name="opls_140" class="HC" element="H" mass="1.00800"  def="H[C;X4]" desc="alkane H"/>
  <Type name="opls_142" class="CM" element="C" mass="12.01100" def="[C;X3](H)(C)C" desc="alkene C (RH-C=)"/>
  <Type name="opls_143" class="CM" element="C" mass="12.01100" def="O" desc="alkene C (H2-C=)"/>
  <Type name="opls_144" class="HC" element="H" mass="1.00800"  def="H[C;X3]" desc="alkene H"/>
  <Type name="opls_145" class="CA" element="C" mass="12.01100" def="O" desc="benzene C"/>
  <Type name="opls_146" class="HA" element="H" mass="1.00800"  def="O" desc="benzene H"/>
 </AtomTypes>

 <!-- Shown below are example entries for force field parameters.

 Please refer to the OpenMM .xml specs for more info:
 http://docs.openmm.org/7.0.0/userguide/application.html#writing-the-xml-file
 <NonbondedForce coulomb14scale="0.833333" lj14scale="0.5">
  <Atom type="opls_135" charge="-0.4157" sigma="0.324999852378" epsilon="0.71128"/>
 </NonbondedForce>

 <HarmonicBondForce>
  <Bond class1="CT" class2="CT" length="0.14" k="392459.1999999999"/>
  <Bond class1="CT" class2="HC" length="0.10800000000000001" k="307105.59999999986"/>
 </HarmonicBondForce>
 <HarmonicAngleForce>
  <Angle class1="CT" class2="CT" class3="HC" angle="2.09439510239" k="669.44"/>
 </HarmonicAngleForce>
 <PeriodicTorsionForce>
  <Proper class1="CT" class2="CT" class3="CT" class4="HC" periodicity1="2" phase1="3.14159265359" k1="15.167"/>
  <Improper class1="CT" class2="CT" class3="CT" class4="CT" periodicity1="2" phase1="3.14159265359" k1="4.6024"/>
 </PeriodicTorsionForce>
 -->
</ForceField>