D2O File Format

The .d2o file format is a binary file type to store densely packed scalar fields. Den2Obj uses this file format to store the data in the relatively unwieldy CHGCAR and PARCHG. In general, .d2o files are about a factor 5 smaller than their corresponding CHGCAR files and can be read in only a fraction of the time because the data does not need to be converted from a human-readable format to binary.

The organization of this file is given in the Table 1.

Table 1: Storage format of the .d2o file.

Position	Type	Length	Description
0x00-0x02	char	3 bytes	Fixed token of “D2O” to identify the file.
0x03-0x06	uint32_t	4 bytes	Protocol identifier token (see below). Selected automatically by default.
0x07-0x2A	float[9]	36 bytes	Unit cell matrix
0x2B-0x36	uint32_t[3]	12 bytes	Grid dimensions (its product is the number of data points)
0x37	uint8_t	1 byte	Floating point bytesize (float = 4, double = 8). Mainly used for validation purposes.
0x38-0x3F	uint64_t	8 bytes	Size of the compressed data stream
0x40..	char[DATASIZE]	DATASIZE bytes	Compressed data stream containing densely packed scalar field.

To convert a CHGCAR file to .d2o file format, run the following command:

den2obj -i CHGCAR -o filename.d2o -t

Note

Den2Obj will automatically look for the best compression algorithm when converting a scalar field to the .d2o format. For the majority of the cases, this corresponds to either the LZMA type of compression or one of the newer array-oriented compression methods.

Protocol tokens

1. GZIP compression

2. LZMA compression

3. BZIP2 compression

4. ZSTD compression

5. Blosc compression