6.2 Input options for VCNEB

The VCNEB method is only enabled with the VASP, GULP and Quantum Espresso codes at the moment.

To switch on the VCNEB mode, you have to:

  1. Specify

    VCNEB : calculationMethod

  2. Create a file Images in the VASP4 format in your folder (VCNEB requires at least two structures, initial and final phases, to run the phase transition pathway prediction).

  3. Specify the following VCNEB options:

$\triangleright $ variable vcnebType

Meaning: Specifies type of the VCNEB calculation. This variable consists of three indices: calculation option, Image number variability, and spring constant variability:

Default: 110

Format:

111 : vcnebType

Note: If vcnebType=111, i.e., a calculation for VCNEB calculation with variable number of Images and variable spring constant is to be performed. We strongly suggest users to run a variable number of Images in VCNEB calculations when investigating the reconstructive phase transitions.

$\triangleright $ variable numImages

Meaning: Initial number of Images to perform the calculation.

Default: 9

Format:

13 : numImages

$\triangleright $ variable numSteps

Meaning: Maximum steps of performing the VCNEB calculation.

Default: 200

Format:

500 : numSteps

Notes: (1) When numSteps=-1, the initial pathway will only be generated without running energy calculations. (2) Convergence of VCNEB pathways is usually rather slow. We recommend to set numSteps to at least 500.

$\triangleright $ variable optReadImages

Meaning: Options for reading the Images file:

Default: 2

Format:

1 : optReadImages

Note: In all options, the initial and final images must be specified. Automatic linear interpolation will be applied to generated the initial Images in option 1 and 2.

$\triangleright $ variable optimizerType

Meaning: Optimization algorithm option of structure relaxation:

Default: 1 (SD) — for VCNEB calculations; 2 (FIRE) — for structure relaxation

Format:

1 : optimizerType

$\triangleright $ variable optRelaxType

Meaning: Structure relaxation mode:

Default: 3

Format:

3 : optRelaxType

$\triangleright $ variable dt

Meaning: Time step for structure relaxation.

Default: 0.05

Format:

0.1 : dt

Note: If dt is very small, the calculations will be very slow. If dt is too large, the calculation will be unstable and often generate meaningless pathways.

$\triangleright $ variable ConvThreshold

Meaning: Halting criteria condition for RMS (Root Mean Square forces) on images.

Default: 0.003 eV/$\text {\r{A}}$

Format:

0.005 : ConvThreshold

$\triangleright $ variable VarPathLength

Meaning: Criterion for path length between Images for variable Image method. When the length between two neighbor images is larger than 1.5 times of VarPathLength, a new image will be added between the two images using linear interpolation; when less then 0.5 the value, the second image will be removed.

Default: The average pathlength between Images of the initial pathway

Format:

0.3 : VarPathLength

$\triangleright $ variable K$\_ $min

Meaning: Minimum spring constant, only used in variable-spring constant VCNEB (in eV/$\text {\r{A}}^2$).

Default: 5

Format:

3 : K$\_ $min

$\triangleright $ variable K$\_ $max

Meaning: Maximum spring constant, only used in variable-spring constant VCNEB (in eV/$\text {\r{A}}^2$).

Default: 5

Format:

6 : K$\_ $max

$\triangleright $ variable Kconstant

Meaning: Spring constant, Only used in fixed-spring constant VCNEB (in eV/$\text {\r{A}}^2$).

Default: 5

Format:

4 : Kconstant

$\triangleright $ variable optFreezing

Meaning: Option for freezing the Image structure. Image structure will be frozen when ConvThreshold is achieved if enabled. Image structure freezing options:

Default: 0

Format:

1 : optFreezing

$\triangleright $ variable optMethodCIDI

Meaning: Option for Climbing-Image (CI) and Downing-Image (DI) method. This method is only suggested to be used when you have a reasonable and well converged pathway. CI/DI-Image method options:

Default: 0

Format:

1 : optMethodCIDI

$\triangleright $ variable startCIDIStep

Meaning: CI/DI method starting step number, only available when optMethodCIDI=1.

Default: 100

Format:

200 : startCIDIStep

$\triangleright $ variable pickupImages

Meaning: Images ID picked up for CI/DI-Image method.

Default: Image ID of transition state and local minimum state Images

Format:

% pickupImages
9 11 17
% EndPickupImages

Note: In this case, the 9$^{th}$, 11$^{th}$ and 17$^{th}$ Images will be picked up for applying CI/DI-Image method. The Image at transition state will be applied with CI-Image method and the Image at local minimum state will be applied DI-Image method automatically.

$\triangleright $ variable FormatType

Meaning: The format of structures in pathway output file, locates at results1/PATH/. Pathway structures output format:

Default: 2

Format:

1 : FormatType

$\triangleright $ variable PrintStep

Meaning: Save the VNCEB restart files locating at results1/STEP/ every PrintStep steps.

Default: 1

Format:

10 : PrintStep

Note: For empirical code, such as GULP, we suggest users to set PrintStep=10 to reduce time cost of saving the restart files.

\includegraphics[scale=1.3]{pic/VCNEB_BH}
Figure 15: The $Ibam$$\rightarrow $$P6/mmm$ transition of BH at 168 GPa 14. A $Pbcm$ intermediate phase is revealed. The saddle points on $Ibam$$\rightarrow $$Pbcm$ and $Pbcm$$\rightarrow $$P6/mmm$ segments have barriers of 0.32 and 0.19 eV/f.u., respectively.

Fig. 15 shows an example of use of the VCNEB method: phase transition mechanism and energy barrier starting from the $Ibam$$\rightarrow $$P6/mmm$ transition of BH system at 168 GPa, we obtained a $Pbcm$ intermediate phase.