Organic molecules basis sets

Flehre W J, Ditchfieid R and Popie J A 1972 Self-consistent molecular-orbital methods XII. Further extension of Gaussian-type basis sets for use in molecular orbital studies of organic molecules J. Chem. Phys. 56 2257-61 Flariharan P C and Popie J A 1973 The influence of polarization functions on molecular orbital hydrogenation energies Theoret. Chim. Acta. 28 213-22... 

The application of density functional theory to isolated, organic molecules is still in relative infancy compared with the use of Hartree-Fock methods. There continues to be a steady stream of publications designed to assess the performance of the various approaches to DFT. As we have discussed there is a plethora of ways in which density functional theory can be implemented with different functional forms for the basis set (Gaussians, Slater type orbitals, or numerical), different expressions for the exchange and correlation contributions within the local density approximation, different expressions for the gradient corrections and different ways to solve the Kohn-Sham equations to achieve self-consistency. This contrasts with the situation for Hartree-Fock calculations, wlrich mostly use one of a series of tried and tested Gaussian basis sets and where there is a substantial body of literature to help choose the most appropriate method for incorporating post-Hartree-Fock methods, should that be desired. 

Another family of basis sets, commonly referred to as the Pople basis sets, are indicated by the notation 6—31G. This notation means that each core orbital is described by a single contraction of six GTO primitives and each valence shell orbital is described by two contractions, one with three primitives and the other with one primitive. These basis sets are very popular, particularly for organic molecules. Other Pople basis sets in this set are 3—21G, 4—31G, 4—22G, 6-21G, 6-31IG, and 7-41G. 

Some of the basis sets discussed here are used more often than others. The STO—3G set is the most widely used minimal basis set. The Pople sets, particularly, 3—21G, 6—31G, and 6—311G, with the extra functions described previously are widely used for quantitative results, particularly for organic molecules. The correlation consistent sets have been most widely used in recent years for high-accuracy calculations. The CBS and G2 methods are becoming popular for very-high-accuracy results. The Wachters and Hay sets are popular for transition metals. The core potential sets, particularly Hay-Wadt, LANL2DZ, Dolg, and SBKJC, are used for heavy elements, Rb and heavier. 

Ah initio methods are accurate and can be reliably applied to unusual structures and inorganic compounds. In most cases, HF calculations are fairly good for organic molecules. Large basis sets should be used. 

The foHowing conclusions apply to organic molecules of about 25 heavy atoms (- 60 atoms total), assuming use of medium-size basis sets (3—2IG ) ... 

Hehre, W. J., Ditchfield, R., Pople, J. A., 1972, Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules J. Chem. Phys., 56, 2257. 

How valid is the minimal-basis assumption that leads from (3.24b) to (3.25) We can examine the accuracy of the NMB restriction (in theNAO basis ) by tabulating the percentage of the total electron density associated with the NMB and NRB sets for some common organic molecules, as shown below ... 

We define here Wlh and W2h theories, respectively, as the modifications of Wl theory for which AVnZ basis sets are only used on elements of groups V, VI, VII, and VIII, but regular VnZ basis sets on groups I, II, III, and IV. (The h stands for heteroatom , as we originally investigated this for organic molecules.) For the purpose of the present paper, we have repeated the validation calculations described in the previous section for Wlh and W2h theories. (For about half of the systems, Wl and Wlh are trivially equivalent.) Some representative results can be found in Table 2.1 for atomization energies/heats of formation, and in Table 2.2 for ionization potentials and electron affinities. 

The article is organized as follows. The main features of the linear response theory methods at different levels of correlation are presented in Section 2. Section 3 describes the calculation of the dipole and quadmpole polarizabilities of two small diatomic molecules LiH and HF. Different computational aspects are discussed for each of them. The LiH molecule permits very accurate MCSCF studies employing large basis sets and CASs. This gives us the opportunity to benchmark the results from the other linear response methods with respect to both the shape of the polarizability radial functions and their values in the vibrational ground states. The second molecule, HF, is undoubtedly one of the most studied molecules. We use it here in order to examine the dependence of the dipole and quadmpole polarizabilities on the size of the active space in the CAS and RASSCF approaches. The conclusions of this study will be important for our future studies of dipole and quadmpole polarizabilities of heavier diatomic molecules. 

A systematic analysis of the electrostatic interactions in the crystals of 40 rigid organic molecules was undertaken by Price and coworkers (D. S. Coombes et al. 1996). In this work, distributed (i.e., local) multipoles up to hexadecapoles, obtained from SCF calculations with 6-31G basis sets, scaled by a factor of 0.9 to allow for the omission of electron correlation, are used in the evaluation of the electrostatic interactions. The experimental lattice constants and structures are reproduced successfully, the former to within a few percent of the experimental... 

As with calculated bond distances, Hartree-Fock bond angles show significant improvement in going from STO-3G to 3-2IG basis sets, and lesser improvement in moving on to 6-3 IG. Either of the latter two Hartree-Fock models appears to be suitable for bond angle calculations on moderate size organic molecules. 

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