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h2.pl
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	#!/usr/bin/perl

	# Usage: h2.pl <nx> <ny> <nz> <rs> > data.h2
	#
	# Generates rectangular lattice of hydrogen atoms, # in each direction = nx,
	# with Wigner radius rs.
	# ...
	# Shifts H atoms in alternating directions so they form H2 molecule
	# starting structures.

	$nx = shift(@ARGV);
	$ny = shift(@ARGV);
	$nz = shift(@ARGV);
	$rs = shift(@ARGV);

	$L = 1.6119919540164693 * $rs; # length of unit cell ((4/3 pi)^(1/3)
	$re = 1.823572; # electron radius

	$dshift = 0.5;

	$idx = 0;
	for ($x = 0; $x < $nx; $x++)
	{
	for ($y = 0; $y < $ny; $y++)
	{
	$dsign = 1;
	for ($z = 0; $z < $nz; $z++)
	{
	$xnuc[$idx] = $x * $L + 0.5 * $L;
	$ynuc[$idx] = $y * $L + 0.5 * $L;
	$znuc[$idx] = $z * $L + 0.5 * $L + $dshift * $dsign;
	$dsign = -$dsign;
	$idx++;
	}
	}
	}

	$numnuc = $idx;

	# Print length of supercell

	printf("Created with h2.pl\n\n");
	printf("%d atoms\n",$numnuc*2);
	printf("2 atom types\n\n");
	printf("%f %f xlo xhi\n", 0, $L * $nx);
	printf("%f %f ylo yhi\n", 0, $L * $ny);
	printf("%f %f zlo zhi\n\n", 0, $L * $nz);
	printf("Masses\n\n");
	printf("1 1.007940\n");
	printf("2 1.000000\n\n");
	printf("Atoms\n\n");

	$j=0;

	# Print out the nuclei and the core electrons
	for ($i = 0; $i < $numnuc; $i++)
	{
	printf("%i %i %f %i %f %f %f %f\n", $j+=1, 1, 1.0, 0, 0.0, $xnuc[$i], $ynuc[$i], $znuc[$i]);
	}

	$spin = 1;
	for ($i = 0; $i < $numnuc; $i++)
	{
	if ($spin == 1) {$spin = -1;} else {$spin = 1;}
	printf("%i %i %f %i %f %f %f %f\n", $j+=1, 2, 0.0, $spin, $re, $xnuc[$i], $ynuc[$i], $znuc[$i]);
	}