Atomic Orbitals, Electron Spin, Linear Combinations

Chapter 1 Atomic Orbitals, Electron Spin, Linear Combinations... 

As we have seen previously (Chapter 5), the eigenfunction for a polyelec-tronic atom is antisymmetric with respect to the exchange of the coordinates of any two electrons, and can be expressed as a Slater determinant whose elements are the various occupied spin-orbitals (or a linear combination of Slater determinants, in the case of open-shell atoms). The same appfies to polyelectronic molecules, the atomic orbitals being replaced by the various occupied molecular orbitals associated with the a and /3 spin-functions spin molecular orbitals. Thus, for the molecules H2O, NH3 or CH4 having five doubly occupied m.o.s (one core s orbital and four valence m.o.s), we have... 

Electron-electron interaction is significant in studies where spin-orbital degeneracy, or near degeneracy, is present. The atomic d-orbitals have a common radial factor and their 15 distinct products give rise to 120 density-density integrals. These are expressed in terms of three basic ones, F0, F2, and F4 in the Slater-Condon formulation. The 100 spin-orbital densities are linear combination of the orbital ones and the 100 by 100 interaction integral matrix has a rank of 15 and is expressed by the Slater-Condon parameters. 

The more accurate Hartree-Fock method approximates the wave function as an antisymmetrized product (Slater determinant or determinants) of one-electron spin-orbitals and finds the best possible forms for the spatial orbitals in the spin-orbitals. Hartree-Fock calculations are usually done by expanding each orbital as a linear combination of basis functions and iteratively solving the Hartree-Fock equations (11.12). The Slater-type orbitals (11.14) are often used as the basis functions in atomic calculations. The difference between the exact nonrelativistic energy and the Hartree-Fock energy is the correlation energy of the atom (or molecule). 

Figure 1.9. Illustration of covalent bonding in graphite. Top the sp linear combinations of. y and p atomic orbitals (detined in Eq. (1.1)). Middle the arrangement of atoms on a plane with B at the center of an equilateral triangle formed by A, A, A" (the arrows connect equivalent atoms) the energy level diagram for the. y, p atomic states, their sp linear combinations (ff and ff) and the bonding (ff) and antibonding ff ) states (up-down arrows indicate electrons spins). Bottom the graphitic plane (honeycomb lattice) and the Ceo molecule.

Figure 1.10. Illustration of covalent bonding in diamond. Top panel representation of the sp linear combinations of s and p atomic orbitals appropriate for the diamond structure, as defined in Eq. (1.3), using the same convention as in Fig. 1.8. Bottom panel on the left side, the arrangement of atoms in the three-dimensional diamond lattice an atom A is at the center of a regular tetrahedron (dashed lines) formed by equivalent B, B, B", B " atoms the three arrows are the vectors that connect equivalent atoms. On the right side, the energy level diagram for the s, p atomic states, their sp linear combinations the bonding (V f) and antibonding ( ) states. The up-down arrows indicate occupation by electrons in the two possible spin states. For a perspective view of the diamond lattice, see Fig. 1.5.

Figure 2. Molecular electronic structure. Linear combinations of N atomic orbitals make N molecular orbitals that are eigenfunctions of an effective 1-electron Hamiltonian. Antisymmetrized products of occupied molecular orbitals times spin functions make configurations. In general, observed states are linear combinations of configurations.

In bond orbital theory the wave functions of the molecule are derived from the wave functions associated with the different bonds of a molecule, i.e. from the bond orbitals. In the LCBO model the molecular orbitals are a linear combination of bond orbitals which in turn are a combination of the atomic orbitals or hybrids forming the bond in question [2b]. In the HLSP-method the many-electron wave functions are a sum of product functions which contain a Heitler and London type of factor (space function and spin function) for each bond of the molecule. 

When the states P1 and P2 are described as linear combinations of CSFs as introduced earlier ( Fi = Zk CiKK), these matrix elements can be expressed in terms of CSF-based matrix elements < K I eri IOl >. The fact that the electric dipole operator is a one-electron operator, in combination with the SC rules, guarantees that only states for which the dominant determinants differ by at most a single spin-orbital (i.e., those which are "singly excited") can be connected via electric dipole transitions through first order (i.e., in a one-photon transition to which the < Fi Ii eri F2 > matrix elements pertain). It is for this reason that light with energy adequate to ionize or excite deep core electrons in atoms or molecules usually causes such ionization or excitation rather than double ionization or excitation of valence-level electrons the latter are two-electron events. 

Consider two well-separated atoms A and B with electron wave functions and which are eigen functions of the atoms, with energies and ei. If we bring these atoms closer, the wave functions start to overlap and form combinations that describe the chemical bonding of the atoms to form a molecule. We will neglect the spin of the electrons. The procedure is to construct a new wave function as a linear combination of atomic orbitals (LCAO), which for one electron has the form... 

When the Hartree-Fock method is applied to molecules, molecular orbitals are used instead of atomic orbitals. To construct the molecular orbitals, one widely used approximation is LCAO (linear combinations of atomic orbitals). According to molecular orbital theory, the total wave function of the system is written as a combination of molecular orbitals, spin functions describing electrons in terms of spin j(a) or — j p). 

In Eq. (2.30), F is the Fock operator and Hcore is the Hamiltonian describing the motion of an electron in the field of the spatially fixed atomic nuclei. The operators and K. are operators that introduce the effects of electrons in the other occupied MOs. Hence, when i = j, J( (= K.) is the potential from the other electron that occupies the same MO, i ff IC is termed the exchange potential and does not have a simple functional form as it describes the effect of wavefunction asymmetry on the correlation of electrons with identical spin. Although simple in form, Eq. (2.29) (which is obtained after relatively complex mathematical analysis) represents a system of differential equations that are impractical to solve for systems of any interest to biochemists. Furthermore, the orbital solutions do not allow a simple association of molecular properties with individual atoms, which is the model most useful to experimental chemists and biochemists. A series of soluble linear equations, however, can be derived by assuming that the MOs can be expressed as a linear combination of atomic orbitals (LCAO)44 ... 

The most simple approach is the Hartree-Fock (HF) self-consistent field (SCF) approximation, in which the electronic wave function is expressed as an antisymmetrized product of one-electron functions. In this way, each electron is assumed to move in the average field of all other electrons. The one-electron functions, or spin orbitals, are taken as a product of a spatial function (molecular orbital) and a spin function. Molecular orbitals are constructed as a linear combination of atomic basis functions. The coefficients of this linear combination are obtained by solving iteratively the Roothaan equations. 

These O, are called Linear Combination of Atomic Orbitals Molecular Orbitals (LCAO MOs) and if they are introduced into the Hartree-Fock equations (eqns (10-2.5)), a simple set of equations (the Hartree-Fock-Roothaan equations) is obtained which can be used to determine the optimum coefficients Cti. For those systems where the space part of each MO is doubly occupied, i.e. there are two electrons in each 0, with spin a and spin respectively so that the complete MOs including spin are different, the total wavefunction is... 

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