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qmc-lttc/vmc_metropolis.c
2021-04-27 20:06:31 +02:00

94 lines
2.2 KiB
C

#include "hydrogen.h"
#include "qmc_stats.h"
void variational_montecarlo(double a, const int nmax, double dt,
double *energy, double *accept) {
int n_accept;
double r_old[3], r_new[3], psi_old, psi_new;
double d_old[3], d_new[3], d2_old, d2_new;
double rnd[3], fact_a, fact_b, sq_dt, q, u;
sq_dt = sqrt(dt);
// Initial position
random_gauss(r_old, 3);
drift(a, r_old, d_old, 3);
d2_old = 0.0;
for (int j = 0; j < 3; ++j) {
d2_old += d_old[j] * d_old[j];
}
psi_old = psi(a, r_old, 3);
*energy = 0.0;
*accept = 0.0;
n_accept = 0;
for (int i = 0; i < nmax; ++i) {
// Compute and accumulate the local energy
*energy += e_loc(a, r_old, 3);
// Compute new position (correct variance of sampled Gaussian)
random_gauss(rnd, 3);
for (int j = 0; j < 3; ++j) {
r_new[j] = r_old[j] + dt * d_old[j] + rnd[j] * sq_dt;
}
// New WF and acceptance probability
drift(a, r_new, d_new, 3);
d2_new = 0.0;
for (int j = 0; j < 3; ++j) {
d2_new += d_new[j] * d_new[j];
}
psi_new = psi(a, r_new, 3);
// Compute the ratio of probabilities q
fact_b = 0.0;
for (int j = 0; j < 3; ++j) {
fact_b += (d_new[j] + d_old[j]) * (r_new[j] - r_old[j]);
}
fact_a = 0.5 * dt * (d2_new - d2_old) + fact_b;
q = psi_new / psi_old;
q = exp(-fact_a) * q * q;
u = (double) rand() / RAND_MAX;
if (u <= q) {
n_accept += 1;
for (int j = 0; j < 3; ++j) {
r_old[j] = r_new[j];
d_old[j] = d_new[j];
}
d2_old = d2_new;
psi_old = psi_new;
}
}
*energy /= nmax;
*accept = (double) n_accept / nmax;
}
int main() {
const double a = 1.2;
const double dt = 1.0;
const long nmax = 1e5;
int nruns = 30;
srand(time(NULL));
double ene[nruns], acc[nruns], obs[2];
for (int i = 0; i < nruns; ++i) {
variational_montecarlo(a, nmax, dt, &ene[i], &acc[i]);
}
ave_error(ene, nruns, obs);
printf("E = %.5lf +/- %.5lf\n", obs[0], obs[1]);
ave_error(acc, nruns, obs);
printf("A = %.5lf +/- %.5lf\n", obs[0], obs[1]);
return 0;
}