 void gpu_nvt_step_two_kernel(gpu_pdata_arrays pdata,
                              unsigned int *d_group_members,
                              unsigned int group_size,
+                             float4 *d_net_force,
                              float Xi,
                              float deltaT)
     {

        pz += vel.z * deltaT;
        
        // read in the image flags
        int4 image = tex1Dfetch(pdata_image_tex, idx);
        
        // time to fix the periodic boundary conditions
        float x_shift = rintf(px * box.Lxinv);

    dim3 grid( (group_size/block_size) + 1, 1, 1);
    dim3 threads(block_size, 1, 1);
    
    // run the kernel
    gpu_nvt_step_two_kernel<<< grid, threads >>>(pdata, d_group_members, group_size, d_net_force, Xi, deltaT);
    
    return cudaSuccess;
    }
