Hexagonal Closest Packing (hcp)

•  2 Vectors of length a and one of length b.
  • Unit cell has 2 sides length a and one side (the 90º one) of length b.
•  Vector Angles of 60º with 90º directions perpendicular to that. 
• This is the other closest packing array 
• The hcp cell has 2 points in the unit cell:
  • one point fully inside                                       =   1
  • 4 apex points have 8 1/3 % inside    4 × .08333 = 1/3
  • 4 apex points have 16 2/3 % inside  4 ×.16666  = 2/3
  •                                                               total   = 2
• Coordination number = 12 (nearest neighbors).