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.
0x03-0x07	uint32_t	4 bytes	Protocol identifier token (see below). Default = 2.
0x08-0x2B	float[9]	36 bytes	Unit cell matrix
0x2C-0x37	uint32_t[3]	12 bytes	Grid dimensions (its product is the number of data points)
0x38	uint8_t	1 byte	Floating point bytesize (float = 4, double = 8). Mainly used for validation purposes.
0x39-0x41	uint64_t	8 bytes	Size of the compressed data stream
0x42..	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 the LZMA type of compression.

Protocol tokens

GZIP compression
LZMA compression
BZIP2 compression