Interaction3Ph Class Reference

Class to calculate the probability rate for one 3-phonon scattering event. More...

#include <interaction_3ph.h>

Public Member Functions
	Interaction3Ph (Crystal &crystal, Eigen::Tensor< double, 5 > &D3, Eigen::MatrixXd &cellPositions2, Eigen::MatrixXd &cellPositions3, Eigen::Tensor< double, 3 > &weights2, Eigen::Tensor< double, 3 > &weights3)
	Default constructor. More...
	Interaction3Ph (const Interaction3Ph &that)
	Copy constructor.
Interaction3Ph &	operator= (const Interaction3Ph &that)
	Assignment operator.
	~Interaction3Ph ()
	Destructor.
std::tuple< std::vector< Eigen::Tensor< double, 3 > >, std::vector< Eigen::Tensor< double, 3 > > >	getCouplingsSquared (const std::vector< Eigen::Vector3d > &q1s_e, const Eigen::Vector3d &q2_e, const std::vector< Eigen::MatrixXcd > &ev1s_e, const Eigen::MatrixXcd &ev2_e, const std::vector< Eigen::MatrixXcd > &ev3Pluss_e, const std::vector< Eigen::MatrixXcd > &ev3Minss_e, const std::vector< int > &nb1s_e, const int nb2, const std::vector< int > &nb3Pluss_e, std::vector< int > &nb3Minss_e)
	Computes the |V3|^2 matrix elements for a bunch of q1 wavevectors at fixed q2 wavevector. More...
void	cacheD3 (const Eigen::Vector3d &q2_e)
	Computes a partial Fourier transform over the q2/R2 variables. More...
int	estimateNumBatches (const int &nq1, const int &nb2)
	Estimate the number of batches that the list of q1 wavevectors must be split into, in order to fit in memory. More...

Detailed Description

In physical notation, this corresponds to the calculation of the term |V3|^2. This class must be schematically used as follow: for (iq2) { cacheD3() int numBatches = coupling3Ph->estimateNumBatches(nq1, nb2); for (batch : batches) { getCouplingsSquared() for ( iq1 : batch ) { ...

The cacheD3() does a partial Fourier transform over the R2 Bravais lattice vector. The list of all q1 wavevectors is split in batches, and the getCouplingSquared() does the Fourier transform over the R1 Bravais lattice vectors for all the wavevectors q1 and (q3 = q1 +- q2) in the batch.

This calculation is GPU optimized. We found that the R1 fourier-transform should be done for a bunch of q1/q3 wavevectors in order to optimize the usage of a GPU (calculations would be too fast for a single q1 wavevector, and most of the time would be spent in data transfer latency). We use Kokkos as the library for GPU acceleration, so that this class can also revert back to a CPU-only calculation if the GPU is not found at compilation time.

The class assumes that the coupling matrix elements in real space passed to input to phoebe are in the form D(R1,R2,R3) -> D(0,R2,R3), i.e. we take advantage of the crystal periodicity to set the phases of R1 to zero, so that a 6-dimensional Fourier transform is enough (and avoid a 9D FT).

Use the environmental variable MAXMEM to set the amount of VRAM in gigabytes available on the GPU/node (depending on the Kokkos installation).

Constructor & Destructor Documentation

Interaction3Ph()

Interaction3Ph::Interaction3Ph	(	Crystal &	crystal,
		Eigen::Tensor< double, 5 > &	D3,
		Eigen::MatrixXd &	cellPositions2,
		Eigen::MatrixXd &	cellPositions3,
		Eigen::Tensor< double, 3 > &	weights2,
		Eigen::Tensor< double, 3 > &	weights3
	)

This method mostly moves data to the GPU if necessary.

Parameters

crystal	crystal object
D3	is a 5-dimensional real tensor with the third-derivative anharmonic force constants. In details, it contains the tensor D3(i,j,k,0,R',R"), where R are Bravais lattice vectors, and i, j, k are indices running over both the Cartesian directions and the basis of atoms in the primitive unit cell. See the theory section for a longer description. The dimensions are (3*numAtoms,3*numAtom,3*numAtom,numBravais,numBravais), where numBravais must be consistent with the cellPositions chosen below.
cellPositions2	Bravais lattice vectors associated to each element in the 4th index of D3 tensor.
cellPositions3	Bravais lattice vectors associated to each element in the 5th index of D3 tensor.
weights2	weight of the Bravais lattice vector, i.e. a counter of the degeneracy of symmetry-equivalent Bravais lattice vectors in cellPositions2. Set to unity (e.g. in ShengBTE) if this is not used.
weights3	weight of the Bravais lattice vector, i.e. a counter of the degeneracy of symmetry-equivalent Bravais lattice vectors in cellPositions3. Set to unity (e.g. in ShengBTE) if this is not used.

Here is the call graph for this function:

Member Function Documentation

cacheD3()

void Interaction3Ph::cacheD3

(

const Eigen::Vector3d &

q2_e

)

Parameters

q2_e	values of the q2 cartesian coordinates over which the Fourier transform is computed.

estimateNumBatches()

int Interaction3Ph::estimateNumBatches	(	const int &	nq1,
		const int &	nb2
	)

Parameters

nq1	total number of q1 wavevectors to be split in batches
nb2	number of bands at the q2 wavevector.

Here is the call graph for this function:

getCouplingsSquared()

std::tuple< std::vector< Eigen::Tensor< double, 3 > >, std::vector< Eigen::Tensor< double, 3 > > > Interaction3Ph::getCouplingsSquared	(	const std::vector< Eigen::Vector3d > &	q1s_e,
		const Eigen::Vector3d &	q2_e,
		const std::vector< Eigen::MatrixXcd > &	ev1s_e,
		const Eigen::MatrixXcd &	ev2_e,
		const std::vector< Eigen::MatrixXcd > &	ev3Pluss_e,
		const std::vector< Eigen::MatrixXcd > &	ev3Minss_e,
		const std::vector< int > &	nb1s_e,
		const int	nb2,
		const std::vector< int > &	nb3Pluss_e,
		std::vector< int > &	nb3Minss_e
	)

Parameters

q1s_e	list of wavevectors at q1 in cartesian coordinates
q2_e	wavevector at q2 in cartesian coordinates
ev1s_e	list of eigenvectors at all the q1 points
ev2_e	eigenvector at q2
ev3Pluss_e	list of eigenvectors at all the q3 = q1 + q2 points
ev3Minss_e	list of eigenvectors at all the q3 = q1 - q2 points
nb1s_e	list of number of bands at q1 @parma nb2: number of bands at q2
nb3Pluss_e	list of number of bands at q3 = q1 + q2
nb3Minss_e	list of number of bands at q3 = q1 - q2

Note: we pass the number of bands as a mechanism to cope with the reduction of bands to only the "active" ones, ,rather than always using all 3*numAtoms phonon bands.