计算弹性常数时INCAR如下：

SYSTEM=SiP
ENCUT=650
ISYM=0

ISTART=0;ICHARG=2
ISMEAR=0;SIGMA=0.1

ISIF=3
IBRION=6
NFREE=4
NSW=1

EDIFF=1E-6
EDIFFG=-0.01
#NPAR=8

LWAVE=FALSE
LCHG=FALSE

IVDW=1
LVDW=.TRUE.

IALGO=48
NELMIN=4
POTIM=0.1

计算过程报警如下： 

VERY BAD NEWS! internal error in subroutine IBZKPT:
 Reciprocal lattice and k-lattice belong to different class of lattices. Often results are still useful...     168

解决办法：

The message does not mean an error.
The procedure doing the evaluation of symmetry uses a numerical criterion (SYMPREC=1E-5)
to determine which atoms are symmetrically equivalent. When this criterion is too soft
the programm can find "false" symmetry and reports that "reciprocal lattice and k-lattice belong
to different class of lattices." To get rid of this message you can:
1) Switch off the symmetry (ISYM=0)
2) Make the treshold for detecting the symmetry more severe (SYMPREC=1E-8)

注：系统有一定的对称性但不太好，但程序的判定条件较低 (SYMPREC=1E-5，缺省？)而认为有对称，

如果设置上面两项，则需要的k点增多！

在INCAR中添加了SYMPREC=1E-8问题得到解决。