Supermolecular Dissociation and Interaction Energies

When using a size-consistent method, the dissociation of homogeneous system such as (HF) into n identical HF monomers [(HF), HF] can be determined by computing the energy of the cluster and the energy of the monomer  

For example, to obtain Eint for the HF HiO- CH3OH trimer requires four separate computations E[FIF], E[Ff20], E[CH30H], as well as the electronic energy of the cluster. The number of computations can be reduced if some of the fragments are identical (e.g., HF HiO HF). 

Interaction Energies Rigid Monomers vs. Fully Optimized Clusters 

The interaction energies reported toward the left side of Table 2 for the HF clusters were calculated with the electronic energies from Table 1 and Eq. [2] (which reduces to Eq. [1] for these homogeneous HF clusters). A simple conversion factor was used to convert Eint from Ej, to kilojoule per mole (lEj, 2625.5 kJmoP ). Step 2 of this tutorial is to calculate the interaction energies on the left side of Table 2. 

Because the RMA had relatively little effect on the electronic energies of the HF clusters (Table 1), it is not surprising that the approximation has only a modest effect on the interaction energies (Table 2). For all three clusters, the magnitude of Ei t and E j decreases slightly when the RMA is employed. Comparison of the values for Ei t from the top of Table 2 to those at the bottom reveals that the change does not exceed 3% [—49.97 vs. —49.13 or 1.7% for (HF)3, —90.53 vs. —88.12 or 2.7% for (HF)4 and —124.53 vs. —120.86 or 2.9% for (HFjf], which is tolerable for many applications. For systems with even smaller cluster-induced geometrical perturbations, such as 71-type van der Waals interactions, the RMA has almost no discernable effect on the inter- 

---