TABLE 1.

Comparison of quantum regions and methods

	DFTB (QM = retinal)a	DFTB (QM = extended)b	DFTB(ext)//DFT(ext)c	DFTB(ext)//DFT(ext+Cl–)d
Planar Ke	10.7	10.5	12.2	12.1
‡(Asp238) barrierf	17.9	18.7	21.2	21.3
‡(Ser115) barrierf	22.8	20.9	24.8	25.1

^aQM region = retinal, optimized with DFTB

^bOpsin = protein including chloride anion and water, but excluding retinal

^cRetinal only

^dInteraction energy between retinal and opsin

^eEnergy relative to the all-trans ground state

^fRate-limiting saddle-point along the 13-cis → all-trans pathways for clockwise (‡Asp²³⁸) or counter-clockwise (‡Ser¹¹⁵) rotation. The barrier is relative to K_p