ATK uses solid harmonics, not spherical ones. The mapping, straight from the source code, is
SolidHarmonicsProperties = {
(0,0) : 's',
(1,-1) : 'y',
(1,0 ) : 'z',
(1,1) : 'x',
(2,-2) : 'xy',
(2,-1) : 'zy',
(2,0) : 'zz-rr',
(2,1) : 'zx',
(2,2) : 'xx-yy',
(3,-3) : 'f(-3)',
(3,-2) : 'f(-2)',
(3,-1) : 'f(-1)',
(3,0) : 'f(0)',
(3,1) : 'f(1)',
(3,2) : 'f(2)',
(3,3) : 'f(3)',
(4,-4) : 'g(-4)',
(4,-3) : 'g(-3)',
(4,-2) : 'g(-2)',
(4,-1) : 'g(-1)',
(4,0) : 'g(0)',
(4,1) : 'g(1)',
(4,2) : 'g(2)',
(4,3) : 'g(3)',
(4,4) : 'g(4)',
}
where the first index is the "l" quantum number and the second "m".