Towards an Atomistic Understanding of Polymorphism in Molecular Solids

Many chemical phenomena are ultimately due to energy balances between atoms. In order to reach firm and clear conclusions one needs a reliable energy decomposition analysis (EDA). The Interacting Quantum Atoms (IQA) energy partitioning method is one of the most recent EDA methods. IQA is a topological energy partitioning that generates well-defined intra- and interatomic contributions, of steric, electrostatic or covalent (exchange) character. IQA has a minimal and powerful architecture and does not suffer from a number of conceptual and practical problems that plague the more traditional non-topological EDAs ( Chem. Soc. Rev. , 44 (2015) 3177). For the first time, our manuscript reports on a protocol for using the IQA to understand polymorphism, which we apply to the three polymorphs of succinic acid (SA), including the unusual polymorph that was recently discovered serendipitously ( CrystEngComm , 20 (2018) 3971). The many intra- and interatomic energy terms from the EDA scheme are processed using a new technique that we developed called the Relative Energy Gradient (REG) method, which clearly identifies the atoms and corresponding energetic terms that govern the behaviour of the total system, in a minimal and unbiased way.