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Administrator<br />

VDW Interactions<br />

The parameters contained in the VDW parameters<br />

table (VDW Interaction.xml) are used to adjust<br />

specific VDW interactions in a molecule, such as<br />

hydrogen bonding, to provide better<br />

correspondence with experimental data in<br />

calculating the MM2 force field.<br />

For example, consider the VDW interaction<br />

between an Alkane carbon (Atom Type 1) and a<br />

hydrogen (Atom Type 5). Normally, the VDW<br />

energy is based on the sum of the VDW radii for<br />

these atoms, found for each atom in the Atom<br />

Types table (1.900Å for Atom type number 1 +<br />

1.400Å for Atom type number 2 = 3.400Å).<br />

However, better correspondence between the<br />

computed VDW energy and experimental data is<br />

found by substituting this sum with the value found<br />

in the VDW Interactions table for this specific<br />

atom type pair (Atom Types 1-5 = 3.340Å).<br />

Similarly, an Eps parameter is substituted for the<br />

geometric mean of the Eps parameters for a pair of<br />

atoms if their atom types appear in the VDW<br />

Interactions table.<br />

Record Order<br />

When sorted by Atom Type, the order of the<br />

records in VDW Interactions table window is as<br />

follows:<br />

Records are sorted by the first atom type number in<br />

the Atom Type field. For example, the record for<br />

Atom Type 1-36 is before the record for atom type<br />

2-21.<br />

For records where the first atom type number is the<br />

same, the records are sorted by the second atom<br />

type number in the Atom Type field. For example,<br />

the record for atom type 2-21 is before the record<br />

for atom type 2-23.<br />

666• Parameter Tables <strong>CambridgeSoft</strong><br />

VDW Interactions

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