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Ab initio quantum chemistry methodsAb initio quantum chemistry methods are computational chemistry methods based on quantum chemistry.^{[1]} The term ab initio indicates that the calculation is from first principles and that no empirical data is used. Robert Parr claims in an interview that the term was first used in letter to him by David Craig and was put into the manuscript of their paper on the excited states of benzene published in 1950.^{[2]} ^{[3]} The simplest type of ab initio electronic structure calculation is the HartreeFock (HF) scheme, in which the instantaneous Coulombic electronelectron repulsion is not specifically taken into account. Only its average effect (mean field) is included in the calculation. This is a variational procedure, therefore the obtained approximate energies, expressed in terms of the system's wave function, are always equal to or greater than the exact energy, and tend to a limiting value called the HartreeFock limit as the size of the basis is increased.^{[4]} Many types of calculations begin with a HartreeFock calculation and subsequently correct for electronelectron repulsion, referred to also as electronic correlation. MøllerPlesset perturbation theory (MPn) and coupled cluster theory (CC) are examples of these postHartreeFock methods.^{[5]} ^{[6]} In some cases, particularly for bond breaking processes, the HartreeFock method is inadequate and this singledeterminant reference function is not a good basis for postHartreeFock methods. It is then necessary to start with a wave function that includes more than one determinant such as Multiconfigurational selfconsistent field and methods have been developed that use these multideterminant references for improvements.^{[5]} Almost always the basis set (which is usually built from the LCAO ansatz) used to solve the Schrödinger equation is not complete, and does not span the Hilbert space associated with ionization and scattering processes (see continuous spectrum for more details). In the HartreeFock method and the Configuration interaction method, this approximation allows one to treat the Schrödinger equation as a "simple" eigenvalue equation of the electronic molecular Hamiltonian, with a discrete set of solutions. Additional recommended knowledge
Classes of methodsThe most popular classes of ab initio electronic structure methods: HartreeFock methods
PostHartreeFock methods
Multireference methods
Example: Is Si_{2}H_{2} like acetylene (C_{2}H_{2})?A series of ab initio studies of Si_{2}H_{2} shows clearly the power of ab initio computational chemistry. They go back over 20 years, and most of the main conclusions were reached by 1995. The methods used were mostly postHartreeFock, particularly Configuration interaction (CI) and Coupled cluster (CC). Initially the question was whether disilyne, Si_{2}H_{2} had the same structure as ethyne (acetylene), C_{2}H_{2}. Slowly (because this started before geometry optimization was widespread), it became clear that linear Si_{2}H_{2} was a transition structure between two equivalent transbent structures and that it was rather high in energy. The ground state was predicted to be a fourmembered ring bent into a 'butterfly' structure with hydrogen atoms bridged between the two silicon atoms. Interest then moved to look at whether structures equivalent to vinylidene  Si=SiH_{2}  existed. This structure is predicted to be a local minimum, i. e. an isomer of Si_{2}H_{2}, lying higher in energy than the ground state but below the energy of the transbent isomer. Then surprisingly a new isomer was predicted by Brenda Colegrove in Henry F. Schaefer, III's group.^{[7]} This prediction was so surprising that it needed extensive calculations to confirm it. It requires post HartreeFock methods to obtain a local minimum for this structure. It does not exist on the HartreeFock energy hypersurface. The new isomer is a planar structure with one bridging hydrogen atom and one terminal hydrogen atom, cis to the bridging atom. Its energy is above the ground state but below that of the other isomers.^{[8]} Similar results were later obtained for Ge_{2}H_{2}.^{[9]} More interestingly, similar results were obtained for Al_{2}H_{2}^{[10]} (and then Ga_{2}H_{2})^{[11]} which has two electrons less than the Group 14 molecules. The only difference is that the fourmembered ring ground state is planar and not bent. The cismonobridged and vinylidenelike isomers are present. Experimental work on these molecules is not easy, but matrix isolation spectroscopy of the products of the reaction of hydrogen atoms and silicon and aluminium surfaces has found the ground state ring structures and the cismonobridged structures for Si_{2}H_{2} and Al_{2}H_{2}. Theoretical predictions of the vibrational frequencies were crucial in understanding the experimental observations of the spectra of a mixture of compounds. This may appear to be an obscure area of chemistry, but the differences between carbon and silicon chemistry is always a lively question, as are the differences between group 13 and group 14 (mainly the B and C differences). The silicon and germanium compounds were the subject of a Journal of Chemical Education article.^{[12]} Accuracy and scalingAb initio electronic structure methods have the advantage that they can be made to converge to the exact solution, when all approximations are sufficiently small in magnitude. In particular configuration interaction where all possible configurations are included (called "Full CI") tends to the exact nonrelativistic solution of the Schrödinger equation. The convergence, however, is usually not monotonic, and sometimes the smallest calculation gives the best result for some properties. The downside of ab initio methods is their computational cost. They often take enormous amounts of computer time, memory, and disk space. The HF method scales nominally as N^{4} (N being the number of basis functions) – i.e. a calculation twice as big takes 16 times as long to complete. However in practice it can scale closer to N³ as the program can identify zero and extremely small integrals and neglect them. Correlated calculations scale even less favorably  MP2 as N^{5}; MP4 as N^{6} and coupled cluster as N^{7}. DFT methods scale in a similar manner to HartreeFock but with a larger proportionality term. Thus DFT calculations are always more expensive than an equivalent HartreeFock calculation. Linear scaling approachesThe problem of computational expense can be alleviated through simplification schemes.^{[13]} In the density fitting scheme, the fourindex integrals used to describe the interaction between electron pairs are reduced to simpler two or threeindex integrals, by treating the charge densities they contain in a simplified way. This reduces the scaling with respect to basis set size. Methods employing this scheme are denoted by the prefix "df", for example the density fitting MP2 is dfMP2 (lowercase is advisable to prevent confusion with DFT). In the local approximation, the molecular orbitals are first localized by a unitary rotation in the orbital space (which leaves the reference wave function invariant, i.e., is not an approximation) and subsequently interactions of distant pairs of localized orbtials are neglected in the correlation calculation. This sharply reduces the scaling with molecular size, a major problem in the treatment of biologicallysized molecules. Methods employing this scheme are denoted by the prefix "L", e.g. LMP2. Both schemes can be employed together, as in the recently developed dfLMP2 and dfLCCSD(T0) methods. In fact, dfLMP2 calculations are faster than dfHartreeFock calculations and thus are feasible in nearly all situations in which also DFT is. Valence bond methodsValence bond (VB) methods are generally ab initio although some semiempirical versions have been proposed. Current VB approaches are^{[1]}:
Quantum Monte Carlo methodsA method that avoids making the variational overestimation of HF in the first place is Quantum Monte Carlo (QMC), in its variational, diffusion, and Green's function forms. These methods work with an explicitly correlated wave function and evaluate integrals numerically using a Monte Carlo integration. Such calculations can be very timeconsuming, but they are probably the most accurate methods known today. See also
References


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ab_initio_quantum_chemistry_methods". A list of authors is available in Wikipedia. 