Displacement Trial Move4a. Examination of Java Code

public void thisTrial(Phase phase) {

double uOld, uNew;

if(phase.atomCount==0) {return;} //no atoms to move

int i = (int)(rand.nextDouble()*phase.atomCount); //pick a random number from 1 to N

Atom a = phase.firstAtom();

for(int j=i; --j>=0; ) {a = a.nextAtom();} //get ith atom in list

uOld = phase.potentialEnergy.currentValue(a); //calculate its contribution to the energy

a.displaceWithin(stepSize); //move it within a local volume

phase.boundary().centralImage(a.coordinate.position()); //apply PBC

uNew = phase.potentialEnergy.currentValue(a); //calculate its new contribution to energy

if(uNew < uOld) { //accept if energy decreased

nAccept++;

return;

}

if(uNew >= Double.MAX_VALUE || //reject if energy is huge or doesn’t pass test

Math.exp(-(uNew-uOld)/parentIntegrator.temperature) < rand.nextDouble()) {

a.replace(); //...put it back in its original position

return;

}

nAccept++; //if reached here, move is accepted

}

Have a look at a simple MC simulation applet

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