problems about the uniaxial strain
Posted: Sat Aug 18, 2012 10:41 am
Hi,
We compiled the VASP again that it can calculate the uniaxial tensile strength. The VASP source routine said that:
At present, VASP does not allow to relax the cellshape selectively
! i.e. for instance only cell relaxation in x direction.
! To be more precisse, this behaviour can not be achived via the INCAR
! or POSCAR file.
! However, it is possible to set selected components of the stress tensor
! to zero.
! The most conveninent position to do this is the routines
! CONSTR_CELL_RELAX (constraint cell relaxation).
! FCELL contains the forces on the basis vectors.
! These forces are used to modify the basis vectors according
! to the following equations:
!
! A_OLD(1:3,1:3)=A(1:3,1:3) ! F90 style
! DO J=1,3
! DO I=1,3
! DO K=1,3
! A(I,J)=A(I,J) + FCELL(I,K)*A_OLD(K,J)*STEP_SIZE
! ENDDO
! ENDDO
! ENDDO
! where A holds the basis vectors (in cartesian coordinates).
!
!-----------------------------------------------------------------------
SUBROUTINE CONSTR_CELL_RELAX(FCELL)
USE prec
REAL(q) FCELL(3,3)
! just one simple example
! relaxation in x directions only
! SAVE=FCELL(1,1)
! FCELL=0 ! F90 style: set the whole array to zero
! FCELL(1,1)=SAVE
RETURN
END SUBROUTINE
So we compile the VASP again with the x,y relaxation as below:
! just one simple example
! relaxation in x,y directions
SAVE=FCELL(1,1)
FCELL=0 ! F90 style: set the whole array to zero
FCELL(1,1)=SAVE
SAVE=FCELL(2,2)
FCELL=0 ! F90 style: set the whole array to zero
FCELL(2,2)=SAVE
RETURN
END SUBROUTINE
In my opinion when the strain applied to the z direction, the x and y direction could be relax until the stress tensor on them are zero. But the results still have stress on the x and y directions, the same as VASP not complied again. And the length of the a,b and c has not been changed. At the same tme, the setting of ISIF (2,3,4) in the INCAR has no effect on the stress showed in the OUTCAR file. Why? Is there any thing wrong about my understanding ?
Any suggestions will be appreciated, thank you.
We compiled the VASP again that it can calculate the uniaxial tensile strength. The VASP source routine said that:
At present, VASP does not allow to relax the cellshape selectively
! i.e. for instance only cell relaxation in x direction.
! To be more precisse, this behaviour can not be achived via the INCAR
! or POSCAR file.
! However, it is possible to set selected components of the stress tensor
! to zero.
! The most conveninent position to do this is the routines
! CONSTR_CELL_RELAX (constraint cell relaxation).
! FCELL contains the forces on the basis vectors.
! These forces are used to modify the basis vectors according
! to the following equations:
!
! A_OLD(1:3,1:3)=A(1:3,1:3) ! F90 style
! DO J=1,3
! DO I=1,3
! DO K=1,3
! A(I,J)=A(I,J) + FCELL(I,K)*A_OLD(K,J)*STEP_SIZE
! ENDDO
! ENDDO
! ENDDO
! where A holds the basis vectors (in cartesian coordinates).
!
!-----------------------------------------------------------------------
SUBROUTINE CONSTR_CELL_RELAX(FCELL)
USE prec
REAL(q) FCELL(3,3)
! just one simple example
! relaxation in x directions only
! SAVE=FCELL(1,1)
! FCELL=0 ! F90 style: set the whole array to zero
! FCELL(1,1)=SAVE
RETURN
END SUBROUTINE
So we compile the VASP again with the x,y relaxation as below:
! just one simple example
! relaxation in x,y directions
SAVE=FCELL(1,1)
FCELL=0 ! F90 style: set the whole array to zero
FCELL(1,1)=SAVE
SAVE=FCELL(2,2)
FCELL=0 ! F90 style: set the whole array to zero
FCELL(2,2)=SAVE
RETURN
END SUBROUTINE
In my opinion when the strain applied to the z direction, the x and y direction could be relax until the stress tensor on them are zero. But the results still have stress on the x and y directions, the same as VASP not complied again. And the length of the a,b and c has not been changed. At the same tme, the setting of ISIF (2,3,4) in the INCAR has no effect on the stress showed in the OUTCAR file. Why? Is there any thing wrong about my understanding ?
Any suggestions will be appreciated, thank you.