Sodium parameters for AM1 and PM3 optimized using a modified genetic algorithm

Sodium is very important as a counterion in biology. However, when used with the most common semiempirical Hamiltonians, such as AM I or PM3, sodium is modeled as a point charge that can accept no electron density, called a "sparkle". To better model sodium, we derived two sets of sodium parameters, which treat sodium on the same footing as other atoms parametrized in semiempirical methods. One set is compatible with the AM I parameter set, while the second is compatible with PM3. These parameters were derived using a modified genetic algorithm with a diverse set of 71 compounds. The average unsigned error for the heats of formation was 10.3 kcal/mol for AMI and 10.5 kcal/mol for PM3.