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Appendix E: Elemental and van der Waals Radii


Standard Elemental Radii

The elemental radii (in Å) listed below are the standard values used to establish the crystal connectivity in the assemblage of CSD entries.

The distance D(A-B) between two atoms A and B is defined to be a bonding distance if:

	R(A) + R(B) - TOL .LE. D(A-B) .LE. R(A) + R(B) + TOL
where:

In some entries the stored value of an elemental radius will differ from the standard value to encompass an unusually long or short bonding situation. Likewise the stored tolerance value may be different from 0.40Å.

EL	R (Å)	EL	R (Å)	EL	R (Å)	EL	R (Å)
Ac	1.88	Er	1.73	Na	0.97	Sb	1.46
Ag	1.59	Eu	1.99	Nb	1.48	Sc	1.44
Al	1.35	F	0.64 	Nd	1.81	Se	1.22
Am	1.51	Fe	1.34	Ni	1.50	Si	1.20
As	1.21	Ga	1.22	Np	1.55	Sm	1.80
Au	1.50	Gd	1.79	O	0.68	Sn	1.46
B	0.83	Ge	1.17	Os	1.37	Sr	1.12
Ba	1.34	H	0.23	P	1.05	Ta	1.43
Be	0.35	Hf	1.57	Pa	1.61	Tb	1.76
Bi	1.54	Hg	1.70	Pb	1.54	Tc	1.35
Br	1.21	Ho	1.74	Pd	1.50	Te	1.47
C	0.68	I	1.40	Pm	1.80	Th	1.79
Ca	0.99	In	1.63	Po	1.68	Ti	1.47
Cd	1.69	Ir	1.32	Pr	1.82	Tl	1.55
Ce	1.83	K	1.33	Pt	1.50	Tm	1.72
Cl	0.99	La	1.87	Pu	1.53	U	1.58
Co	1.33	Li	0.68	Ra	1.90	V	1.33
Cr	1.35	Lu	1.72	Rb	1.47	W	1.37
Cs	1.67	Mg	1.10	Re	1.35	Y	1.78
Cu	1.52	Mn	1.35	Rh	1.45	Yb	1.94
D	0.23	Mo	1.47	Ru	1.40	Zn	1.45
Dy	1.75	N	0.68	S	1.02	Zr	1.56
						X	0.00

The element symbol X is used ocasionally when a site is (equally) occupied by two elements. The radius assigned to X is then often the mean of the two elemental radii.

van der Waals Radii

The default van der Waals radii are taken from A. Bondi (1964) "van der Waals Volumes and Radii" J.Phys.Chem. 68, 441-451. These values are shown in the table below (all values in Å).

If an element does not appear in the table it is assigned a value of 2.0Å.

Ag	1.72	Ar	1.88	As	1.85	Au	1.66
Br	1.85	C	1.70	Cd	1.58	Cl	1.75
Cu	1.40	F	1.47	Ga	1.87	H	1.20
He	1.40	Hg	1.55	I	1.98	In	1.93
K	2.75	Kr	2.02	Li	1.82	Mg	1.73
N	1.55	Na	2.27	Ne	1.54	Ni	1.63
O	1.52	P	1.80	Pb	2.02	Pd	1.63
Pt	1.72	S	1.80	Se	1.90	Si	2.10
Sn	2.17	Te	2.06	Tl	1.96	U	1.86
Xe	2.16	Zn	1.39				

If the van der Waals radii and/or tolerance are to be changed from the standard values then the VDW-RAD command in the 3D-CONSTRAIN sub-menu must be used before the CONTACT command is defined (see Section 3.2 Ex.5).

It should be noted that these van der Waals radii are provided merely as a starting point for non-bonded searches. Indeed, the revision of van der Waals radii is a vitally important area of research (see Nyburg and Faerman (1985) "A revision of van der Waals atomic radii for molecular crystals" Acta Crystallogr. B41, 274-279).


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