In a photoionization experiment, several different ionization energies can be measured, depending on the degree of vibrational excitation of the cations. In general, the following two types of ionization energies are considered:

1.Adiabatic ionization energy, E_Ia
The energy corresponding to the transition
M(X, v” = 0) + hnà M⁺(x, v’ = 0) + e^-
that is, the minimum energy required to eject an electron from a molecule in its ground vibrational state and transform it into a cation in the lowest vibrational level of an electronic state x of the cation.

2.Vertical ionization energy, E_Iv
The vertical ionization energy corresponds to the transition
M(X, v” = 0) + hnà M⁺(x, v’ = n) + e^-
where, the value n of the vibrational quantum number v’ corresponds to the vibrational level whose wavefunction gives the largest overlap with the v” = 0 wavefunction. This is the most probable transition and usually corresponds to the vertical transition where the internuclear separations of the ionic state are similar to those of the ground state.

It is clear that transitions to each of the ionic states x,a,b, c, … will have individual adiabatic and vertical ionization energies.

1垂直电离能计算：优化分子构型，（同时也就得到分子能量了）使用这个构型减少电子计算离子的能量，离子减分子能量就是电离能了。

垂直电离能的计算原理：电离后的分子（实际上已是离子，但还没来得及发生结构松弛）的结构还没来得及改变时体系的能量与电离前分子的能量的差值。

2垂直电子亲和能：在中性几何构型下阴离子和分子的能量差；

AIP = E(阳离子，优化结构) - E(中性分子，优化结构)
VIP = E(阳离子，优化中性分子结构) - E(中性分子，优化结构)
ADE = E(中性分子，优化结构) - E(阴离子，优化结构)
VDE = E(中性分子，优化阴离子结构) - E(阴离子，优化结构)

http://w3.iams.sinica.edu.tw/lab/wbtzeng/

http://chemmat.hubu.edu.cn:82/Teach/jpkc/wulihuaxue/web/duomeiti/%E7%BB%93%E6%9E%84%E5%8C%96%E5%AD%A6%E8%AE%B2%E4%B9%89/new_page_22.htm