where ψ and ψ* are the molecular wavefunctions for the bonding and antibonding molecular orbitals, respectively, ψa and ψb are the atomic wavefunctions from atoms a and b, respectively, and ca and cb are adjustable coefficients. these coefficients can be positive or negative, depending on the energies and symmetries of the individual atomic orbitals. as the two atoms become closer together, their atomic orbitals overlap to produce areas of high electron density. so, molecular orbitals are formed between the two atoms. the atoms are held together by the electrostatic attraction between the positively charged nuclei and the negatively charged electrons occupying bonding molecular orbitals.