Table 1. Comparison of results for NH₃ from a multipole refinement with constrained hydrogen parameters, an X-ray constrained wavefunction (XCW) fitting on the fixed final geometry from the multipole refinement, and an iterative Hirshfeld atom refinement (HAR) without any constraints: χ² agreement statistics, N—H bond length (Å), fractional coordinates and ADPs (Ų).

	Multipoles	XCW	HAR
χ2	0.8	†	0.6
r(N—H)	1.010	1.010	0.987 (5)
N x	0.2103 (1)	0.2103	0.21059 (5)
H x	0.3722	0.3722	0.3668 (9)
H y	0.2627	0.2627	0.269 (1)
H z	0.1113	0.1113	0.1148 (8)
N U 11	0.0372 (2)	0.0372	0.0363 (2)
N U 12	−0.0009 (1)	−0.0009	−0.00163 (9)
H U 11	0.0053	0.0053	0.070 (3)
H U 22			0.066 (3)
H U 33			0.064 (3)
H U 12			−0.016 (3)
H U 13			0.006 (2)
H U 23			−0.009 (3)

^†χ² values before and after X-ray constrained wavefunction fitting.