2. Essential glossary

Throughout this documentation, we employ terms and abbreviations which may not be immediately clear to the new user. Herein is a compiled list of important terms accompanied by brief descriptions. Further discussion for many of these terms is included in the user documentation.

2.1. Cluster models

In order to take into account the environmental energetical contributions to an excited molecule inside a crystal, we offer several kinds of non-intrusive embedding methods:

ONIOM

Our own N-layered Integrated molecular Orbital and Molecular mechanics. An extrapolative (subtractive) embedding paradigm for multilevel calculations[6][16][11]

EC

Embedded Cluster. The application of the ONIOM method with electrostatic embedding to a cluster of molecules taken from their crystalline positions

EEC

Ewald Embedded Cluster. Similar to EC but using an electrostatic embedding scheme aimed at reproducing the Ewald potential of the crystal

SC-EEC

Self-Consistent Ewald Embedded Scheme. The EEC model where the embedding charges are computed self consistently

Mechanical embedding

ONIOM embedding where no point charges are included and the intersystem electrostatic interaction is purely at low level

Electrostatic embedding

ONIOM embedding where the point charges from the surrounding cluster are included in the Hamiltonian of the high level of theory to be treat Coulombic terms in the excited state

2.2. Calculations

In the expression of the 2-level ONIOM energy, the system under scrutiny is partitioned into two regions:

Model system

The central molecule(s) whose properties are under scrutiny

Real system

The complete cluster of molecules taken from their crystalline positions, including and surrounding the model system

By nature, 2-level ONIOM combines two levels of theory:

High level

A level of theory which describes the required properties of the model system. In photochemistry this will often be an excited state method

Low level

A less computationally expensive level of theory than the high level. This will typically be a ground state method

When optimising the geometry of a molecule using one of the cluster models, a few concurrent calculations must be carried out.

mh

Model system high level calculation

ml

Model system low level calculation

rl

Real system low level calculation

mg

Model system calculation in the ground state of the high level

2.3. General photochemistry

MECI

Minimum Energy Conical Intersection. The minimum energy point in the crossing seam between potential energy surfaces. This point is located in fromage by using the penalty function method.[15]

Exciton coupling

A measure of the splitting of the excited state energy levels due to the formation of a molecular dimer[4]