[Wien] Answer to one trivial question

[Michael Fischer]

Hello,

the conversion formulae to go from the hexagonal setting to the rhombohedral setting are:

a_rhomb = (3 * a_hex^2 + c_hex^2)^(1/2) / 3

sin (alpha_rhomb/2) = 3 / (2 * (3 + (c_hex/a_hex)^2)^(1/2))

And the other way 'round:

a_hex = 2 * a_rhomb * sin(alpha_rhomb/2)

c_hex = a_rhomb * (3 + 6 * cos(alpha_rhomb))^(1/2)

I hope I did not include any typos...

Greetings
Michael
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