The Madelung Constant of Organic Salts

The Madelung constant is a key feature determining the lattice energy of a crystal structure and hence its stability. However, the complexity of the calculation has meant that it has previously not been readily available for complex structures, for example for organic salts. We propose a new robust method for calculating Madelung constants of such structures based on a generalized numerical direct summation approach, The method is applied to various organic salts from the ionic liquid and pharmaceutical fields. The values calculated are seen to be a unique feature of the crystal structure, reflecting the positioning of the ions in the unit cell and being sensitive to ion pairing. The difference in Madelung constants between different polymorphs of a compound is also shown.