Expression

lmfit also provides an expression parser for use in fitting models. Consequently, the expression parser can be also used in SpectraFit for generating fits with constraint conditions or global fits.

JSON

"peaks": {
    "1": {
        "pseudovoigt": {
            "amplitude": {
                "max": 2,
                "min": 0,
                "vary": true,
                "value": 1
            },
            "center": {
                "max": 2,
                "min": -2,
                "vary": true,
                "value": 0
            },
    },
    "2": {
        "pseudovoigt": {
            "amplitude": {
                "expr": "pseudovoigt_amplitude_1"
            },
            "center": {
                "expr": "pseudovoigt_center_1 + 1.68"
            }
        }
    }
}

In this particular use case, the amplitude of the second peak is set to the amplitude of the first peak; the center of the second peak is set to the center of the first peak plus 1.68.

In this example, the expression parser is used to set the amplitude of the peak 1 and 2 to equally, as well as the center of peak 2 to the center of peak 1 plus 1.68.

The right notation of peaks

In contrast to the json, yaml, or toml file, the peaks are written in the following way, that number of the peak is the last number.

`toml`	Definition
number.function.attribute	function_attribute_number