Sum of an array element using OpenMP

Question

I am trying to parallelize the following nested "for loops" (in C) using OpenMP.

for (dt = 0; dt <= maxdt; dt++) {
  for (t0 = 0; t0 <= nframes-dt; t0++) {
    for (i=0; i<natoms; i++) {
      VAC[dt] = VAC[dt] + dot_product(vect[t0][i],vect[t0+dt][i]) ;
    }
  }
}

Basically this calculates an auto-correlation function of a time dependent vector (vect). I need the VAC array as the final output using OpenMP.

I have tried using the reduction sum approach of OpenMP to perform this, by adding the following line above the innermost loop (for (i=0; i<natoms; i++)).

#pragma omp parallel for default(shared) private(i,axis) schedule(guided) reduction(+: VAC[dt])

But this does not work, since reduction sum does not work for arrays. What would be the best and most efficient way to parallelize such codes? Thanks.