Table 1. RE of the DMP-L⁺ and DMP-D⁺ states obtained using various computational methods.

Method	RE (eV)
HF	−0.53
MP2	0.81
DFT
B3LYP	—a
M06	— a
M06-2X	— a
M06-HF	— a
PBE0	— a
BHandHLYP	0.19
PZ-SIC	0.34
CCSDb	0.23
MP2_CCSD(T)-SP	0.39
CCSD_CCSD(T)-SP	0.38
Experiment	0.33 (0.04)

CCSD, coupled cluster method with single and double excitations; DFT, density functional theory; DMP, N,N'-dimethylpiperazine; HF, Hartree–Fock; PZ-SIC, Perdew and Zunger self-interaction correction; RE, relative energy.

Single-point energy calculations were carried out with the CCSD(T) method, in one case using a structure obtained with MP2 and in the other case using a structure obtained with CCSD.

Zero point energy correction has not been applied but an estimate based on ground vibrational states would reduce the calculated relative energy by 0.07 eV, see Supplementary Note 3. As the molecule is at high temperature, the full, ground vibrational state zero point correction is an overestimate.

^aNo value shown because the DMP-L⁺ state was not stable at this level of theory.

^bThe CCSD optimizations were carried out with the aug-cc-pVDZ basis set because calculations with the cc-pVTZ were found to be too demanding for our computational resources.