International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 9.7, pp. 897-899

## Section 9.7.1.2. Symmorphism and antimorphism

A. J. C. Wilson,a V. L. Karenb and A. Mighellb

aSt John's College, Cambridge CB2 1TP, England, and bNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

#### 9.7.1.2. Symmorphism and antimorphism

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Wilson (1993d) classified the space groups by degree of symmorphism. A fully symmorphic space group contains only the syntropic' symmetry elements and a fully antimorphic space group contains only the antitropic' elements The remaining symmetry elements are atropic'. The two triclinic space groups, P1 and , contain only atropic' elements, and are thus not classified by these criteria. The rest are divided into five groups, in accordance with the balance of symmetry elements within the unit cell. For the 71 non-triclinic space groups symmorphic in the strict sense (Wilson, 1993d; Subsection 1.4.2.1 ), the classification gives:

 (1) Fully symmorphic (only syntropic elements): 14. (2) Tending to symmorphism (mainly syntropic elements): 28. (3) Equally balanced (equal numbers of syntropic and antitropic elements): 20. (4) Tending to antimorphism (mainly antitropic elements): 9. (5) Fully antimorphic (only antitropic elements): 0.

The distribution of the 230 space groups (11 enantiomorphic couples merged) by arithmetic crystal class and degree of symmorphism is given in Table 9.7.1.2.

 Table 9.7.1.2| top | pdf | Space groups arranged by arithmetic crystal class and degree of symmorphism (Wilson, 1993d), as frequented by homomolecular structures with one molecule in the general position (in superscript numerals; according to Belsky, Zorkaya & Zorky, 1995)
 (a) Triclinic, monoclinic and orthorhombic systems. The triclinic space groups are a special case, with degree of symmorphism' undefined, and they are not assigned to any particular column. For *, † see Subsection 9.7.4.1.
Arithmetic crystal classFully symmorphicTending to symmorphismEqually balancedTending to antimorphismFully antimorphic
1P *P1†(90)
*P(1796)
2P *P2(0) P21(1327)
2C *C2 †(109)
mP *Pm(0) Pc(58)
mC *Cm(0) Cc(144)
2/mP *P2/m(0) P21/m(0) P21/c(5951)
P2/c(11)
2/mC *C2/m(0) C2/c(587)
222P *P222(0) P2221(0) P21212(30) P212121(2795)
222C *C222(0) C2221(11)
222F *F222(0)
222I *I222(0)
I212121(0)
mm2P *Pmm2(0) Pma2(0) Pmc21(0) Pca21(153)
Pcc2(1) Pna21(367)
Pnc2(1)
Pmn21(0)
Pba2(1)
Pnn2(1)
mm2C *Cmm2(0) Cmc21(0)
Ccc2(0)
2mmC *Amm2(0) Abm2(0)
Ama2(0)
Aba2† (11)
mm2F     *Fmm2(0) Fdd2†(35)
mm2I *Imm2(0) Iba2†(14)
Ima2(0)
mmmP *Pmmm(0) Pccm(0) Pnnn(0) Pnna(1) Pbca(827)
Pmma(0) Pban(0) Pcca(3)
Pmna(0) Pbam(0)
Pmmn(0) Pccn(37)
Pbcm(0)
Pnnm(0)
Pbcn(60)
Pnma(0)
mmmC *Cmmm(0) Cmma(0) Cmcm(0)
Cmca(0)
Cccm(0)
Ccca(0)
mmmF *Fmmm(0) Fddd(2)
mmmI *Immm(0) Ibam(0)
Ibca(0)
Imma(0)
 (b) Tetragonal space groups. For *, † see Subsection 9.7.4.1.
Arithmetic crystal classFully symmorphicTending to symmorphismEqually balancedTending to antimorphismFully antimorphic
4P *P4(0) P42(1) P41,3(40)
4I I41(3) *I4(3)
P *P(0)
I *I(7)
4/mP *P4/m(0) P42/m(0) P4/n(1) P42/n(20)
4/mI *I4/m(0) I41/a†(29)
422P *P422(0) P4212(0) P41,3212†(49)
P4222(0) P41,322(1) P42212(1)
422I I4122†(0) *I422(0)
4mmP *P4mm(0) P4bm(0) P42cm(0)
P42nm(0)
P4cc(0)
P4nc(0)
P42mc(0)
P42bc(1)
4mmI *I4mm(0)
I4cm(0)
I41md(0)
I41cd(5)
2mP *P2m(0) P2c(0) P21c(12)
P21m(0)
m2P *Pm2(0) Pc2(0)
Pb2(0)
Pn2(0)
m2I *Im2(0) Ic2†(0)
2mI *I2m(0) I2d(0)
4/mmmP *P4/mmm(0) P4/mcc(0) P4/nbm(0)
P42/mmc(0) P4/nmm(0) P4/nnc(0)
P42/mcm(0)   P4/mbm(0)
P4/mnc(0)
P4/ncc(0)
P42/nbc(0)
P42/nnm(0)
P42/mbc(0)
P42/mnm(0)
P42/nmc(0)
P42/ncm(0)
4/mmmI *I4/mmm(0) I4/mcm(0)
I41/amd(0)
I41/acd(0)
 (c) Trigonal space groups. For *, † see Subsection 9.7.4.1.
Arithmetic crystal classFully symmorphicTending to symmorphismEqually balancedTending to antimorphismFully antimorphic
3P *P3(0) P31,2(33)
3R *R3†(11)
P *P(1)
R *R(30)
312P 321P *P312(0) *P321(0) P31,212†(0) P31,221†(10)
32R *R32†(0)
3m1P 31mP *P3m1(0) *P31m(0) P3c1†(0) P31c(0)
3mR *R3m(0) R3c(7)
m1P 1mP *Pm1(0) *P1m(0) Pc1†(0) P1c(0)
mR *Rm(0) Rc(0)
 (d) Hexagonal space groups. For *, † see Subsection 9.7.4.1.
Arithmetic crystal classFully symmorphicTending to symmorphismEqually balancedTending to antimorphismFully antimorphic
6P *P6(0) P62,4(1) P63(0) P61,5(22)
P *P(0)
6/mP *P6/m(0) P63/m(0)
622P *P622(0) P62,422(0) P6322(1) P61,522†(2)
6mmP *P6mm(0) P6cc(0)
P63cm(0) P63mc(0)
m2P 2mP *Pm2(0) *P2m(0) Pc2†(0) P2c(0)
6/mmmP *P6/mmm(0) P6/mcc(0)
P63/mcm(0) P63/mmc(0)
 (e) Cubic space groups. For *, †, see Subsection 9.7.4.1. No examples with one molecule in general position were found, so the frequencies are omitted.
Arithmetic crystal classFully symmorphicTending to symmorphismEqually balancedTending to antimorphismAntimorphic except for 3
23P *P23 P213†
23F *F23†
23I *I23 I213†
mP *Pm Pn Pa
mF *Fm Fd
mI *Im Ia
432P *P432 P4232† P41,332†
432F *F432 F4132†
432I *I432 I4132†
3mP *P3m P3n
3mF *F3m F3c
3mI *I3m I3d
mmP *Pmm Pmn Pnm Pnn
mmF *Fmm Fmc Fdm Fdc
A few points about the symmorphic groups are worth noting. The 14 fully symmorphic' space groups are those that (i) have primitive cells and (ii) have no secondary or tertiary axes (three each monoclinic, orthorhombic, tetragonal, hexagonal; two trigonal; no cubic). Secondary axes, even though syntropic in the conventional space-group notation, generate additional antitropic axes in accordance with the principles set out by Bertaut (2005, Chap. 4.1 ). These additional axes are not indicated in the full' Hermann–Mauguin space-group symbol, but should appear in the extended' symbol (Bertaut, 2005, Table 4.3.2.1 4). As a result of the additional axes, 21 symmorphic space groups with primitive cells are shifted to the tending to symmorphism' column (five tetragonal, six trigonal, five hexagonal, five cubic). Lattice centring has a similar or greater effect; the 36 centred symmorphic space groups are spread over the three columns tending to symmorphism' (seven), equally balanced' (20) and tending to antimorphism' (nine).
From the nature of the definitions, no symmorphic space group can be fully antimorphic'. The 14 groups under the latter heading consist of (i) 12 space groups with no special positions (Subsection 9.7.4.1), and (ii) two space groups whose only special positions have symmetry (Subsection 9.7.4.2). On these criteria, the two triclinic groups excluded from discussion would fall naturally into the column fully antimorphic'. The remaining space groups have no obvious outstanding characteristics. Most of them fall under the heading tending to antimorphism', though there are some in each of the columns tending to symmorphism' and `equally balanced'.