Why HDF5?

It is portable and platform independent. Once they are generated, they can be used anywhere.

Basic strategy

• Same for serial and parallel executions. 
• H5Group represents a physical entity or concept. 
• Multiple instances of a datatype are represented by H5Group and enumerated. E.g., state_0, ..., state_99 for 100 states. 
• Avoid using high-dimensional arrays. 
• High-dimensionality due to physical concepts, such as spin, band and k-point, should be handled by HDF5 Group. 
• Different codes use different index schemes ( band index, spin index, k-point etc) but the fast index in any language and any reasonable code means the same: 3D grid with three indexes or plane-wave vectors with one index. 
• Repacking to a higher-dimensional array is inefficient (CPU and memory). 
• Tools and libraries can handle any indexing scheme without requiring modification in application codes. 
• Visualization tools can display data easily if an array is mapped on a regular grid. 

ESHDF schema

• The memory layout of a multi-dimensional array follows C convention. 
• The boolean type uses integer and follows C convention, i.e., 1 for true. 
• Complex type is represented by extending the dimension of an array, e.g., psi_r[n0][n1][n2][2] for complex wavefunctions. 
• To reduce the size of a file, recommend NOT to have psi_r 
• This is a default behavior of pw2qmcpack and wfconvert tools.