The Scope of Quantum Mechanical Calculations for Polymers

Ab initio quantum mechanical (QM) calculations represent approximate efforts to solve the Schrodinger equation, which describes the electronic structure of a molecule based on the Born-Oppenheimer approximation (in which the positions of the nuclei are considered fixed). It is typical for most of the calculations to be carried out at the Hartree—Fock self-consistent field (SCF) level. The major assumption behind the Hartree-Fock method is that each electron experiences the average field of all other electrons. Ab initio molecular orbital methods contain few empirical parameters. Introduction of empiricism results in the various semiempirical techniques (MNDO, AMI, PM3, etc.) that are widely used to study the structure and properties of small molecules. 

Application of quantum chemical methods to polymeric systems is complicated. The computer hardware capabilities to deal with real polymers are, at this point, inadequate. In general, the CPU requirements grow with the fourth to sixth power of the number of functions used and the system Hamiltonian does not normally contain information about the solvent. The major limitation is the length of time that can be simulated. Important dynamic polymer properties are associated with relaxation times that are many orders of magni- 

Quantum mechanical methods are very useful for study of small fragments (such as parts of oligomers, or even in some cases, the whole oligomer) of polymers. Most of these techniques have been optimized for small molecules and, in some cases, for well-characterized large molecules. Properties of interest to polymer scientists that may be calculated by these techniques include  

Free energies of conformers in relatively steep potential wells 

Validation of potential energy functions for molecular mechanics 

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