Wavefunctions, computation

We will run a series of state-averaged CAS(4,4) calculations (the option is StateAverago) on the ground state structure, using this sequence of basis sets 3-21G, 4-31G, 6-31G(d) and 6-31+G(d,p). Begin the first computation using the wavefunction computed in the RHF calculation. 

As illustrated also in other Chapters of this book (see especially Chapters 3), the transformation from the adiabatic wavefunctions computed by ah initio methods to diabatic ones, is often the first task to be accomplished when approaching the study of the dynamics around a CI. The main historical reason for the preference given to the diabatic representation was the problem of the divergence of the coupling between adiabatic surfaces, as illustrated for example in the review of Ref. 1. This computational difficulty seems to have been overcome, judging from the increasing... 

In the PCM-EOM-CC/SACCI approximation the excited electronic states are represented by a linear (Cl-like) expansion build-up on the coupled-cluster wavefunction for the ground state [6-8]. We use the PTE couple-cluster wavefunction, computed in the presence of the frozen Hartree-Fock reaction field, as it leads to a more simpler and physically transparent PCM-EOM theory. The EOM-CC theory leads to a non-Hermitian eigenvalue problem with right and left eigenvalues. 

PEF employs partial atomic charges (identical for related atom types) selected from a Mulliken population analysis of a minimal basis set wavefunction computed for a-D-Glcp. Although the relatively low magnitude of these partial atomic charges renders them unsuitable for applications with common models for explicit water, the PEF422 parameters were optimized for modeling isolated carbohydrates, in vacuo, with a dielectric constant (e) of 3 Debye. ... 

One fomis a reference wavefunction T (this can be of the SCF, MPn, CC, etc variety) tlie energy differences are computed relative to the energy of this fiinction. 

Kurst G R, R A Stephens and R W Phippen 1990. Computer Simulation Studies of Anisotropic iystems XIX. Mesophases Formed by the Gay-Berne Model Mesogen. Liquid Crystals 8 451-464. e F J, F Has and M Orozco 1990. Comparative Study of the Molecular Electrostatic Potential Ibtained from Different Wavefunctions - Reliability of the Semi-Empirical MNDO Wavefunction. oumal of Computational Chemistry 11 416-430. 

The first- and second- order RSPT energy and first-order RSPT wavefunction correction expressions form not only a useful computational tool but are also of great use in understanding how strongly a perturbation will affect a particular state of the system. By... 

The algorithms of the mixed classical-quantum model used in HyperChem are different for semi-empirical and ab mi/io methods. The semi-empirical methods in HyperChem treat boundary atoms (atoms that are used to terminate a subset quantum mechanical region inside a single molecule) as specially parameterized pseudofluorine atoms. However, HyperChem will not carry on mixed model calculations, using ab initio quantum mechanical methods, if there are any boundary atoms in the molecular system. Thus, if you would like to compute a wavefunction for only a portion of a molecular system using ab initio methods, you must select single or multiple isolated molecules as your selected quantum mechanical region, without any boundary atoms. 

Solution Running a Sfable calculation indicates that the computed UHF wavefunction for triplet molecular oxygen is stable ... 

Cl density method, which uses analytic derivatives of the wavefunction to compute the dipole moments, resulting in much more accurate predictions, as is illustrated in this case. You can request the Cl density by including either DensityaCI or DensityaCurrenI in the route section of a Cl-Singles calculation, n... 

The Isodensity PCM (IPCM) model defines the cavity as an isodensity surface of the molecule. This isodensity is determined by an iterative process in which an SCF cycle is performed and converged using the current isodensity cavity. The resultant wavefunction is then used to compute an updated isodensity surface, and the cycle is repeated until the cavity shape no longer changes upon completion of the SCF. 

It has thus been established that the only difficulty which exists in the evaluation of the energy and wavefunction of any molecule. .. is the amount of computing necessary. 

As computational facilities improve, electronic wavefunctions tend to become more and more complicated. A configuration interaction (Cl) calculation on a medium-sized molecule might be a linear combination of a million Slater determinants, and it is very easy to lose sight of the chemistry and the chemical intuition , to say nothing of the visualization of the results. Such wavefunctions seem to give no simple physical picture of the electron distribution, and so we must seek to find ways of extracting information that is chemically useful. 

Taxonomic classification invariantly have been first (and frequently the only until now) constructive influence of quantum theory on a given area in chemistry, and numerical approximations to wavefunctions have been remarkably unproductive compared with the enormous amount of mathematical quantities handled by modern computers (Jorgensen [1986]). 

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