# The compute gkma keyword

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## Contents

## Purpose

The `compute_gkma`

keyword is used to calculate the modal heat current using the Green-Kubo Modal Analysis (GKMA) method [Lv 2016].

## Grammar

compute_gkma sample_interval first_mode last_mode bin_option size

`sample_interval`

is the sampling interval (in number of steps) used to compute the heat modal heat current.`first_mode`

is the first mode in the eigenvector.in file to include in the calculation.`last_mode`

is the last mode in the eigenvector.in file to include in the calculation.`bin_option`

determines which binning technique to use:`bin_size`

or`f_bin_size`

.`size`

defines how the modes are added to each bin. If`bin_option`

=`bin_size`

, then this is an integer describing how many modes are included per bin. If`bin_option`

=`f_bin_size`

, then binning is by frequency and this is a float describing the bin size in THz.

## Examples

### Example 1

compute_gkma 10 1 27216 f_bin_size 1.0

This means that (1) you want to calculate the modal heat current with the GKMA method; (2) the modal heat flux will be sampled every 10 steps; (3) the range of modes you want to include of calculations are from 1 to 27216; (4) you want to bin the modes by frequency with a bin size of 1 THz.

### Example 2

compute_gkma 10 1 27216 bin_size 1

This example is identical to Example 1, except the modes are binned by count. Here, each bin only has one mode (i.e. all modes are output).

### Example 3

compute_gkma 10 1 27216 bin_size 10

This example is identical to Example 2, except each bin has 10 modes.

## Output file

## Caveats

- This computation can be very memory intensive. The memory requirements are comparable to the size of the eigenvector.in file.
- Depending number of steps run, sampling interval, and number of bins, the heatmode.out output file can become very large as well (i.e. many GBs)
- This keyword cannot be used in the same run as the compute_hnema keyword. The keyword appeared last will be used in the run.

## References

- [Lv 2016] Wei Lv and Asegun Henry,
*Direct calculation of modal contributions to thermal conductivity via Green-Kubo modal analysis*, New J. Phys.**18**, 013028 (2016).