/** integrate Newton's equation by one step, velocity Verlet */
void vv() {
double dth = dt * .5, dtl = dt / L;
for (int id = 0; id < N; id++) // velocity-verlet, first half
for (int d = 0; d < D; d++) {
v[id][d] += f[id][d] * dth;
x[id][d] += v[id][d] * dtl;
}
force(); // compute force
for (int id = 0; id < N; id++) // velocity-verlet, second half
for (int d = 0; d < D; d++) v[id][d] += f[id][d] * dth;
rmcom(); // rm angular momentum however isn't good idea due to
// pbc and minimal image convension
K = vrescale(0.02); // compute kinetic energy and adjust velocity
E = K + Us;
step++;
avU.add(U);
avK.add(K);
avp.add(p);
}
void clearData() {
step = 0;
avU.clear();
avK.clear();
avp.clear();
}