Structures of the water trimer, main electronic configurations and corresponding CI coefficients (cut-off = 0.05) obtained from CASPT2(4,8) and (12,12) calculations^a.

Structure		CASPT2(n,m)	S1		S0
			Conf.	CI coeff.	Conf.	CI coeff.
G3-[1]		(4,8)		0.9872	Ψ 0	0.9865
			—	—		0.1025
		(12,12)		0.9743	Ψ 0	0.9745
			—	—	—	—
E3-[1]‡		(4,8)		0.9855	Ψ 0	0.9820
				0.0986		0.0597
		(12,12)		0.9641	Ψ 0	0.9618
				0.0660	—	—
E3-[2]		(4,8)		0.9782	Ψ 0	0.9722
				0.0938		0.0970
		(12,12)		0.9639	Ψ 0	0.9596
				0.0888		0.0812
E3-[4]		(4,8)		0.9641	Ψ 0	0.9515
				0.2171		0.1899
		(12,12)		0.9463	Ψ 0	0.9279
			—	—	—	—

^a Ψ ^r and C^r_a = a → r singly excited determinant (S-type) and CI coefficient and; Ψ^r,s_a,b and C^r,s_a,b = a → r and b → s doubly excited determinant (D-type) and CI coefficient; the indices a and b, and r and s label occupied and virtual or unoccupied spin orbitals, respectively; a bar or lack of a bar is to denote beta (β) and alpha (α) spin orbitals, respectively.