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If one phase exhibits PO, a simple statement is necessary:
GEWICHT=SPHARx
x
determines the order of the spherical harmonics model and can
be 0, 2, 4, 6, 8 or 10. Except the triclinic lattice there are constraints
between the parameters of the model. BGMN itself determines these constraints
automatically.
Lets have a look at the gypsum structure file, lines 5 and 6:
PHASE=Gypsum SpacegroupNo=15 HermannMauguin=I12/a1 // PARAM=A=0.6522_0.645^0.658 PARAM=B=1.5202_1.51^1.53 // PARAM=C=0.5679_0.56^0.575 PARAM=BETA=118.4_118^119 // RP=3 PARAM=B1=0_0^0.008 // GEWICHT=SPHAR6 GOAL:gypsum=GEWICHT // LIMIT2=3*2 LIMIT4=3*4 LIMIT6=3*8 E=CA+2 Wyckoff=e TDS=0.0088 y=0.4213 E=S Wyckoff=e TDS=0.0066 y=0.9216 E=O-2 Wyckoff=f TDS=0.0149 x=0.5505 y=0.1319 z=0.9632 E=O-2 Wyckoff=f TDS=0.0141 x=0.6671 y=0.0223 z=0.7582 E=O-2 Wyckoff=f TDS=0.0219 x=0.4588 y=0.1821 z=0.3796There is the
GEWICHT=SPHAR6
statement in line 5 which determines
the use of a 6th order spherical harmonics model. Additionally the
statements LIMITx=...
are included on line 6. These statements
are required for another feature of BGMN, the automatic
reduction of the PO model.
If the phase content is too low, the order of the model will be reduced down
to SPHAR0
(isotropy), if necessary. Thats why you can
use the same structure file for all analytical problems even if nearly nothing
of the phase is contained in the sample.