Constant pressure engines are added to ohmms and they implement general Langevin TPN equations of motion based on "Langevin dynamics in constant pressure extended systems", D. Quigley and M. I. J Prbert, J. Chem. Phys. V120, 11432 (2004). By choosing the parameters, the algorithm can work as EPN (constant energy, pressure), TPN (constant temperature, pressure) and steepest decent structural minimizations (see SD minimization).
The main input file of this section is hcpomega-y.tpn.xml.
<?xml version="1.0" encoding="utf-8"?>
<simulation>
<keyword> Ti 360 atom hcp-omega interface along Y MEAM </keyword>
<project id="hcpomega-y.tpn" series="0">
<application name="ohmms" role="md" version="0.2.3.1"/>
</project>
<random parallel="true" seed = "-1"/>
<process>
<particlesets>
<particleset src="hcpomega-y.xml" />
</particlesets>
<mdsim>
<particlelayout method="N2" scale="1.1"/>
<potential name="MEAM">
<filename>MEAMPOT.Ti.308a</filename>
</potential>
<ensemble name="tpn" engine="TPNLangevin">
<parameter name="temperature" condition="K">1000</parameter>
<parameter name="timestep" condition="fsec">1</parameter>
<parameter name="volfreq" condition="rad/fsec">3.05e-4</parameter>
<parameter name="friction" condition="fsec-1">0.005</parameter>
<parameter name="volfriction" condition="fsec-1">0.005</parameter>
<parameter name="pressure" condition="kB">0</parameter>
</ensemble>
<loop blocks="1" steps="1000">
<record name="ensemble" stride="1" />
<record name="particle" format="ascii" stride="1">
<attrib name="position" datatype="xmolArray"></attrib>
</record>
<record name="particle" format="xml" stride="10" condition="overwrite" />
</loop>
</mdsim>
</process>
</simulation>
The name of engine (class) is TPNLangevin. The friction parameter,
<parameter name="friction" condition="fsec">0.05</parameter>
works the same way for TVNLangevin engine. To control the pressure dynamics, two parameters are introduced: volfreq and volfriction (consult the paper by Quigley for the details). The values of volfreq and volfriction need to be calibrated by running simulations.