Import ABACUS++G_8 code; cleanup files; new README

src/NRG/NRG_K_Weight_integrand.cc

@@ -0,0 +1,47 @@
+/**********************************************************
+
+This software is part of J.-S. Caux's ABACUS library.
+
+Copyright (c).
+
+-----------------------------------------------------------
+
+File:  NRG_K_Weight_integrand.cc
+
+Purpose:  for NRG, this function specifies the momentum-dependent
+          part of the perturbation prefactor.
+          Collaboration with Robert Konik.
+
+***********************************************************/
+#include "JSC.h"
+
+using namespace std;
+using namespace JSC;
+
+namespace JSC {
+
+  // This function defines the integrand for momentum-dependent perturbation prefactor
+  DP K_Weight_integrand (Vect_DP args)
+  {
+    // Momentum-dependent part of weight used in selecting kept form factors.
+    // This function gives the integrand, with the weight being
+    // \int_{-1/2}^{1/2} dx integrand(x)
+
+    DP value;
+
+    { // Case of a harmonic trap of strength omega_trap centered on 0:
+
+      // args[0] == x
+      // args[1] == iK
+      // args[2] == L
+      // args[3] == omega_trap
+
+      value = 12.0 * args[0] * args[0] * cos(twoPI * args[0] * args[1]) * pow(args[2], 2.0) * pow(args[3], 3.0);
+    }
+
+    return(value);
+  }
+
+
+
+} // namespace JSC