(c) (i) Explain how to solve the secular equation of the Hartree-Fock scheme in the knowledge that the Fock matrix depends on the solutions of this equation. (4 marks) (ii) What is the core idea behind density functional theory? (3 marks) (iii) For a two-electron system, prove that a Slater determinant of spin orbitals obeys the Pauli principle. (4 marks) (d) State and briefly discuss three differences and three similarities between Hckel theory and Hartree-Fock theory. (11 marks)