/// \file
/// \ingroup tutorial_graphics
/// \notebook
/// Plot the Amplitude of a Hydrogen Atom.
///
/// Visualize the Amplitude of a Hydrogen Atom in the n = 2, l = 0, m = 0 state.
/// Demonstrates how TH2F can be used in Quantum Mechanics.
///
/// The formula for Hydrogen in this energy state is \f$ \psi_{200} = \frac{1}{4\sqrt{2\pi}a_0 ^{\frac{3}{2}}}(2-\frac{\sqrt{x^2+y^2}}{a_0})e^{-\frac{\sqrt{x^2+y^2}}{2a_0}} \f$
///
/// \macro_image
/// \macro_code
///
/// \author Advait Dhingra

#include <cmath>

double WaveFunction(double x, double y) {
   double r = sqrt(x *x + y*y);

   double w = (1/pow((4*sqrt(2*TMath::Pi())* 1), 1.5)) * (2 - (r / 1)*pow(TMath::E(), (-1 * r)/2)); // Wavefunction formula for psi 2,0,0

   return w*w; // Amplitude

}

void schroedinger_hydrogen() {
   TH2F *h2D = new TH2F("Hydrogen Atom",
                        "Hydrogen in n = 2, l = 0, m = 0 state; Position in x direction; Position in y direction",
                        200, -10, 10, 200, -10, 10);

   for (float i = -10; i < 10; i += 0.01) {
      for (float j = -10; j < 10; j += 0.01) {
         h2D->Fill(i, j, WaveFunction(i, j));
      }
   }

   gStyle->SetPalette(kCividis);
   gStyle->SetOptStat(0);

   TCanvas *c1 = new TCanvas("c1", "Schroedinger's Hydrogen Atom", 750, 1500);
   c1->Divide(1, 2);

   auto c1_1 = c1->cd(1);
   c1_1->SetRightMargin(0.14);
   h2D->GetXaxis()->SetLabelSize(0.03);
   h2D->GetYaxis()->SetLabelSize(0.03);
   h2D->GetZaxis()->SetLabelSize(0.03);
   h2D->SetContour(50);
   h2D->Draw("colz");

   TLatex *l = new TLatex(-10, -12.43, "The Electron is more likely to be found in the yellow areas and less likely to be found in the blue areas.");
   l->SetTextFont(42);
   l->SetTextSize(0.02);
   l->Draw();

   auto c1_2 = c1->cd(2);
   c1_2->SetTheta(42.);

   TH2D *h2Dc = (TH2D*)h2D->Clone();
   h2Dc->SetTitle("3D view of probability amplitude;;");
   h2Dc->Draw("surf2");
}