13 Fragment-Based Methods

13.7 The Second-Generation ALMO-EDA Method

The ALMO-EDA method introduced in Section 13.5 is a very useful tool for unraveling the nature of intermolecular interactions. Nevertheless, it has two major shortcomings: (i) Although the polarization (POL) energy is variationally evaluated, it does not have a meaningful basis set limit. As the employed basis set becomes larger, the POL term starts to be contaminated by charge transfer (CT) and loses its intended meaning. (ii) The frozen (FRZ) interaction is a monolithic term in the original ALMO-EDA scheme. In practice, further decomposition of the FRZ term is often desired. For example, if one wants to use ALMO-EDA as a tool for the development of empirical force fields, the separation of the FRZ term into contributions from permanent electrostatics, Pauli repulsion and dispersion will be helpful since they are usually modeled by distinct functional forms in classical force fields. These drawbacks have been addressed recently, defining the second generation of the ALMO-EDA method (also referred to as “EDA2" in the text below).390, 392, 393