Potential energy surfaces can be of two types: adiabatic or diabatic. Adiabatic surfaces are defined within the Born–Oppenheimer approximation by the energy (eigenvalue) of a given solution to the electronic Schrodinger equation at each geometry. Such solutions are obtained by using the full electronic Hamiltonian, that is, including kinetic energy, Coulomb, scalar relativistic and spin–orbit terms. Diabatic surfaces can be defined as generated from the eigenvalues of the Schrodinger equation solved using a Hamiltonian from which one or more terms have been omitted; in the present case, the spin–orbit coupling terms. Surfaces of both types are shown for a notional spin-forbidden reaction in Fig. 1. Reactants are on diabatic surface 1 (e.g. corresponding to a triplet state), with products on surface 2 (e.g. a singlet). The corresponding minima have different geometries, and the surfaces cross at the Minimum Energy Crossing Point (MECP).
势能面分为绝热和非绝热两种。绝热势能面是在波恩-奥本海默近似的框架内定义的，指的是在每个几何构型下，电子薛定谔方程的本征值，即能量。上述方程解对应全电子哈密顿算符，即包括电子动能，库仑，标量相对论和旋-轨各项。非绝热势能面可以由使用省略某（些）项的哈密顿的薛定谔方程的本征值解获得；这里省略的是旋-轨项。图一展示自旋禁阻反应模型的两种势能面。反应物在非绝热势能面1上（如，代表一个三重态），产物在势能面2上（如，某个单重态）。相对应的能量最小值处于不同的几何构型，势能面相交于最小能量交叉点（MECP）处。
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