Pseudo-orbitals

Here 1/ is the effective potential and a>i i is a nodeless pseudo-orbital that can be derived from Xi, in several different ways. For first-row atoms, Christiansen, Lee and Pitzer (1979) suggest... 

So, for some match point / to infinity, the atomic pseudo-orbital is identical to the valence HF atomic orbital. For radial distances less than / the pseudoorbital is defined by a polynomial expansion that goes to zero. The values of the polynomial are found by matching the value and first three derivatives of the HF orbital at / . 

Recently Thiel and Voityuk have constructed a workable NDDO model which also includes d-orbitals for use in connection with MNDO, called MNDO/d. With reference to the above description for MNDO/AM1/PM3, it is clear that there are immediately three new parameters Cd, Ud and (dd (eqs. (3.82) and (3.83)). Of the 12 new one-centre two-electron integrals only one (Gjd) is taken as a freely varied parameter. The other 11 are calculated analytically based on pseudo-orbital exponents, which are assigned so that the analytical formulas regenerate Gss, Gpp and Gdd. 

The behaviour of the frontier electrons was also attributed to a certain type of electron delocalization between the reactant and the reagent 40). A concept of pseudo-n-orbital was introduced by setting up a simplified model, and the electron delocalization between the 71-electron system of aromatic nuclei and the pseudo-orbital was considered to be essential to aromatic substitutions. The pseudo-orbital was assumed to be built up out of the hydrogen atom AO attached to the carbon atom at the reaction center and the AO of the reagent species, and to be occupied by zero, one, and two electrons in electrophilic, radical, and nucleophilic reactions. A theoretical quantity called "superdelocalizability was derived from this model. This quantity will be discussed in detail later in Chap. 6. 

In the present work, correlation consistent basis sets have been developed for the transition metal atoms Y and Hg using small-core quasirelativistic PPs, i.e., the ns and (nA)d valence electrons as well as the outer-core (nA)sp electrons are explicitly included in the calculations. This can greatly reduce the errors due to the PP approximation, and in particular the pseudo-orbitals in the valence region retain some nodal structure. Series of basis sets from double-through quintuple-zeta have been developed and are denoted as cc-pVwZ-PP (correlation consistent polarized valence with pseudopotentials). The methodology used in this work is described in Sec. II, while molecular benchmark calculations on YC, HgH, and Hg2 are given in Sec. III. Lastly, the results are summarized in Sec. IV. 

It is clear from Eq. (207) that the pseudization only turns on for the valence orbitals and has a null effect on the core orbitals. More interestingly, the exact orbital energies are not altered. Similar to Eq. (7), one can define the valence pseudo-density in terms of the valence pseudo-orbitals ... 

The action of the projection operator (-e +A) 0 ><0 is to raise the eigenvalue of the core orbital % to the value A. A new lower bound for the eigenvalue for the pseudo-orbital xl can be shown to be the lower of A and ej. In practice the core eigenvalues are usually shifted so as to be degenerate with the lowest valence eigenvalues of the same symmetry. The coefficients a in equation (37) can now assume values which allow the pseudo-orbital Xt to be nodeless and thus capable of representation by a smaller basis set expansion. 

Having reviewed the theoretical background to the core-valence separation, we now turn to the practical implementation of the theory. Starting from equations (31)— (34) we note that the valence pseudo-orbitals are eigenfunctions of an equation which can be written as... 

Table 1 The generation of a 4s pseudo-orbital of the bromine atom by adding a linear combination of core orbitals to the 4s SCF orbital...

Figure 1 A comparison between the radial dependence of (a) the near-Hartree-Fock 4s orbital of bromine and (b) a pseudo-orbital obtained as a linear combination of the 4s orbital with the core s-orbitals (see Table 1)...

We assume that each nucleon has a pseudo-spin i and pseudo-orbital angular momentum k. These couple to form the single particle angular momenta J,J (in [j]) of the two interacting nucleons. The wavefunction of a pair of nucleons coupled to a total angular momentum L (and z component p) is then given by ... 

The first term has a triplet CO moiety, while the second exhibits spin alternation on the carbonyl fragment. Since each interaction between two electrons of the same spin account for one Pauli repulsion, there are two Pauli repulsion in the first term of P and only one in the second (the interaction between the spins on CH3 and O has a longer distance and is discounted). Hyperconjugation would have the effect of delocalizing the pseudo-orbital of methyl on the neighboring carbon, or, equivalently of adding the following two VB structures to P ... 

Teichteil et al.41 fit a spin-orbit pseudo-operator such that its action on a pseudo-orbital optimally reproduces the effect of the true spin-orbit operator on the corresponding all-electron orbital. Ermler, Ross, Christiansen, and co-workers42 9 and Titov and Mosyagin50 define a spin-orbit operator as the difference between the and j dependent relativistic effective pseudopotentials (REPs)51... 

C-H bond is replaced by calculating the difference in correlation energy between a are then replaced by a set of nodeless pseudo-orbitals. The regular valence orbitals will... 

Mitroy et al. (1984) carried out an extensive configuration-interaction calculation of the structure amplitude (q/ 0) for correlated target and ion states. The long-dashed curve in fig. 11.7(a) shows their momentum distribution multiplied by 2. They found that the dominant contribution came from the pseudo-orbital 3d, calculated by the natural-orbital transformation. Pseudo-orbitals are localised to the same part of space as the occupied 3s and 3p Hartree—Fock orbitals and therefore contribute to the cross section at much higher momenta than the diffuse Hartree—Fock 3d and 4d orbitals. The measurements show that the 4d orbital has a larger weight than is calculated by Mitroy et al, who overestimate the 3d component. 

Fig. 1. Effective potentials for / = 0 light solid line) and I = 1 (heavy solid line) for xenon. The 5p (squares) and 6s (circles) pseudo-orbital amplitudes are also shown.

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