text: #include <iostream>
  #include <Eigen/Core>
  
  using namespace Eigen;
  
  #ifndef SCALAR
  #define SCALAR float
  #endif
  
  #ifndef SIZE
  #define SIZE 10000
  #endif
  
  #ifndef REPEAT
  #define REPEAT 10000
  #endif
  
  typedef Matrix<SCALAR, Eigen::Dynamic, 1> Vec;
  
  using namespace std;
  
  SCALAR E_VDW(const Vec &interactions1, const Vec &interactions2)
  {
    return (interactions2.cwise()/interactions1)
           .cwise().cube()
           .cwise().square()
           .cwise().square()
           .sum();
  }
  
  int main() 
  {
    //
    //          1   2   3   4  ... (interactions)
    // ka       .   .   .   .  ...
    // rab      .   .   .   .  ...
    // energy   .   .   .   .  ...
    // ...     ... ... ... ... ...
    // (variables
    //    for
    // interaction)
    //
    Vec interactions1(SIZE), interactions2(SIZE); // SIZE is the number of vdw interactions in our system
    // SetupCalculations()
    SCALAR rab = 1.0;  
    interactions1.setConstant(2.4);
    interactions2.setConstant(rab);
    
    // Energy()
    SCALAR energy = 0.0;
    for (unsigned int i = 0; i<REPEAT; ++i) {
      energy += E_VDW(interactions1, interactions2);
      energy *= 1 + 1e-20 * i; // prevent compiler from optimizing the loop
    }
    cout << "energy = " << energy << endl;
  }
  