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Note that special atom types are defined for carbon atoms involved in small rings, such as cyclopropane and cyclobutane. The reason for this will be discussed in Section 2.2.2.

Note that special atom types are defined for carbon atoms involved in small rings, such as cyclopropane and cyclobutane. The reason for this will be discussed in Section 2.2.2.

same grouping holds for other feature as well, e.g. all C=O bonds are approximately 1.22A long and have vibrational frequencies of approximately 1700 cm-1, all double-bonded carbons are essentially planar, etc. The transferability also holds for energetic features. A plot of the heat of formation for linear alkanes, i.e. CH3(CH2)nCH3, against the chain length n produces a straight line, showing that each CH2 group contributes essentially the same amount of energy. (For a general discussion of estimating heat of formation from group additivities, see Benzon.3)

The picture of molecules being composed of structural units ("functional groups") that behave similarly in different molecules forms the very basis of organic chemistry. Molecular structure drawings, where alphabetic letters represent atoms and lines represent bonds, are used universally. Organic chemists often build ball and stick, or CPK space-filling, models of their molecules to examine their shapes. Force field methods are in a sense a generalization of these models, with the added feature that the atoms and bonds are not fixed at one size and length. Furthermore, force field calculations enable predictions of relative energies and barriers for interconversion of different conformations.

The idea of molecules being composed of atoms, which are structurally similar in different molecules, is implemented in force field models as atom types. The atom type depends on the atomic number and the type of chemical bonding it is involved in. The type may be denoted with either a number or a letter code. In MM2, for example, there are 71 different atom types (type 44 is missing). Type 1 is an sp3-hybridized carbon, and an sp2-hybridized carbon may be type 2, 3 or 50, depending on the neighbour atom(s). Type 2 is used if the bonding is to another sp2-carbon (simple double bond), type 3 is used if the carbon is bonded to an oxygen (carbonyl group) and type 50 is used if the carbon is part of an aromatic ring with delocalized bonds. Table 2.1 gives a complete list of the MM2(91) atom types, where (91) indicates the year when the parameter set was released. The atom type numbers roughly reflect the order in which the corresponding functional groups were parameterized.

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