optimize the Vashishta potentials

The analytical and tabulated Vashishta potentials are optimized. 

Speeds (Tesla K40; beta-SiC with 8000 atoms at 300 K and 0 GPa) 
before the optimization:
-- Analytical: 2.9*1.0e6 atom*step/second
-- Tabulated: 4.6*1.0e6 atom*step/second (table length = 20 000; relative force error = 1.0e-6)
after the optimization:
-- Analytical: 3.3*1.0e6 atom*step/second
-- Tabulated: 6.1*1.0e6 atom*step/second (table length = 20 000; relative force error = 1.0e-6)

src/common.h

@@ -311,6 +311,7 @@ typedef struct
     real *thermo;           // some thermodynamic quantities
     real *b; real *bp;      // for bond-order potentials
     real *fv; real *fv_all; // for SHC calculations
+    real *f12x, *f12y, *f12z; // partial force for many-body potentials
 } GPU_Data;
 
 

src/finalize.cu

@@ -74,6 +74,14 @@ void finalize(Force_Model *force_model, CPU_Data *cpu_data, GPU_Data *gpu_data)
         CHECK(cudaFree(force_model->vas_table.table));
     }
 
+    // for SW and Tersoff type potentials
+    if (force_model->type >= 30)
+    {
+        CHECK(cudaFree(gpu_data->f12x));
+        CHECK(cudaFree(gpu_data->f12y));
+        CHECK(cudaFree(gpu_data->f12z));
+    }
+
     // Free the major memory allocated on the CPU
     MY_FREE(cpu_data->NN);
     MY_FREE(cpu_data->NL);

src/potential.cu

@@ -837,6 +837,20 @@ void process_potential
         CHECK(cudaMalloc((void**)&gpu_data->b,  memory)); 
         CHECK(cudaMalloc((void**)&gpu_data->bp, memory)); 
     }
+
+    // for SW and Tersoff type potentials
+    if (force_model->type >= 30)
+    {
+        // Assume that there are at most 20 neighbors for the many-body part;
+        // This should be more than enough
+        // I do not change 20 to para->neighbor.MN because MN can be very large
+        // in some cases
+        int memory = sizeof(real) * para->N * 20; 
+        CHECK(cudaMalloc((void**)&gpu_data->f12x, memory));
+        CHECK(cudaMalloc((void**)&gpu_data->f12y, memory));
+        CHECK(cudaMalloc((void**)&gpu_data->f12z, memory));
+    }
+
 }