Optimize HIP pair-list handling for CDNA LJPME

- Use bitwise prefix accounting when storing sparse interactions as single pairs in the HIP pair-list kernel. This reduces the number of ballot operations needed to compute per-lane single-pair offsets.
- For HIP CDNA single precision, raise MAX_BITS_FOR_PAIRS to 8 so more sparse interactions are emitted as single pairs instead of full tiles. Keep the existing double precision and RDNA thresholds unchanged.
- Also simplify the HIP LJPME direct correction by computing alpha^2*r2

platforms/common/src/kernels/coulombLennardJones.cc

@@ -41,8 +41,12 @@
 #if DO_LJPME
     // The multiplicative term to correct for the multiplicative terms that are always
     // present in reciprocal space.
+#if defined(USE_HIP)
+    const real dar2 = EWALD_DISPERSION_ALPHA*EWALD_DISPERSION_ALPHA*r2;
+#else
     const real dispersionAlphaR = EWALD_DISPERSION_ALPHA*r;
     const real dar2 = dispersionAlphaR*dispersionAlphaR;
+#endif
     const real dar4 = dar2*dar2;
     const real dar6 = dar4*dar2;
     const real invR2 = invR*invR;

platforms/hip/src/HipNonbondedUtilities.cpp

@@ -549,11 +549,9 @@ void HipNonbondedUtilities::createKernelsForGroups(int groups) {
             }
             else {
                 if (context.getSIMDWidth() > 32) {
-                    // CDNA
-                    if (context.getNumAtoms() < 100000)
-                        maxBits = 4;
-                    else // Large systems
-                        maxBits = 0;
+                    // CDNA wave64 GPUs benefit from storing more sparse
+                    // interactions as single pairs.
+                    maxBits = 8;
                 }
                 else {
                     // RDNA

platforms/hip/src/kernels/findInteractingBlocks.hip

@@ -553,10 +553,10 @@ extern "C" __global__ __launch_bounds__(GROUP_SIZE) void findBlocksWithInteracti
                 if (__ballot(storeAsSinglePair)) {
                     unsigned int sum = 0;
                     unsigned int prevSum = 0;
-                    for (int i = 1; i <= MAX_BITS_FOR_PAIRS; i++) {
-                        warpflags b = __ballot(interactCount == i);
-                        sum += warpPopc(b) * i;
-                        prevSum += warpPopc(b&warpMask) * i;
+                    for (int bit = 1; bit <= MAX_BITS_FOR_PAIRS; bit <<= 1) {
+                        warpflags b = __ballot(storeAsSinglePair && ((interactCount & bit) != 0));
+                        sum += warpPopc(b) * bit;
+                        prevSum += warpPopc(b&warpMask) * bit;
                     }
                     unsigned int pairStartIndex = 0;
                     if (indexInWarp == 0)