I was trying to construct matrix elements for helium atom to get the electronic energy spectrum. I used 1s, 2s, and the three 2p orbitals to construct the two body basis. I treated the electron as fermion with no spin degree of freedom. Hence , I have 10 two body slater determinant.I used those basis (Slater determinant)to calculate the matrix elements of the hamiltonian. However, the two electron integral is not converging (Monte carlo method was used) and the contructed matrix is not hermitian ?