Since you're submitting a paper to a journal that has certain formatting requirements for the paper's bibliography (and lots of other aspects of the paper too, no doubt!), you should ask (if you haven't already done so) for a BibTeX bibliography style (.bst) file that implements the journal's requirements. With any luck, this .bst file will be compatible with natbib and also implement the journal's house style (of making you cite the names of all authors of pieces that have three or fewer authors).
If you're not that lucky, there are three possibilities. First, if the .bst file is not compatible with natbib, you're obviously out of luck. (Fortunately, though, natbib is enormously robust and works with the overwhelming majority of .bst files.) Second, if the .bst file is compatible with natbib but does not contain a function called format.lab.names, you're also out of luck. That's what's meant, basically, by the statement in natbib's manual that "starred [citation command] versions can only list the full authors if the .bst file supports this feature." (Of course, such a .bst file must also provide a few functions that call the format.lab.names function...)
Third, if the .bst file does contain such a function but doesn't implement the journal's house style, i.e., if the \cite[pt]* macros output Adams et~al. for a piece that has exactly three authors (and the first author's surname is "Adams"...), you are not out of luck. All you need to do is to replace the existing format.lab.names function with one that does obey the journal's house style. The existing format.lab.names function should (more or less...) look like:
FUNCTION {format.lab.names}
{ 's :=
"" 't :=
s #1 "{vv~}{ll}" format.name$
s num.names$ duplicate$
#2 >
{ pop$
" " * bbl.etal *
}
{ #2 <
'skip$
{ s #2 "{ff }{vv }{ll}{ jj}" format.name$ "others" =
{
" " * bbl.etal *
}
{ bbl.and space.word * s #2 "{vv~}{ll}" format.name$
* }
if$
}
if$
}
if$
}
You should replace this code with the following code:
FUNCTION {format.lab.names}
{'s :=
"" 't :=
#1 'nameptr :=
s num.names$ 'numnames :=
numnames 'namesleft :=
{ namesleft #0 > }
{ s nameptr
"{vv~}{ll}" format.name$
't :=
nameptr #1 >
{
nameptr #2 =
numnames #3 > and
{ "others" 't :=
#1 'namesleft := }
'skip$
if$
namesleft #1 >
{ ", " * t * }
{
s nameptr "{ll}" format.name$ duplicate$ "others" =
{ 't := }
{ pop$ }
if$
t "others" =
{
" " * bbl.etal *
}
{
numnames #2 >
{ "," * }
'skip$
if$
bbl.and
space.word * t *
}
if$
}
if$
}
't
if$
nameptr #1 + 'nameptr :=
namesleft #1 - 'namesleft :=
}
while$
}
After making this replacement, save the .bst file under a new name and adjust the \bibliographystyle command to point to the new file.
If you get complaints from BibTeX about nonexistent bbl.and and/or bbl.etal functions, just add the following code to the new .bst file (somewhere near the top of the file, soon after the start of the section in which the bibtex functions are defined):
FUNCTION {bbl.and}
{ "and"}
FUNCTION {bbl.etal}
{ "et~al." }
You used \\ after the subsection heading to start a new line. Instead, please insert a blank line to start a new paragraph. This will solve your problem. (The way headings are defined in the MDPI class file allows you to use \\ there to start a new line. In other document classes you'd receive an error message when using \\ after a heading.)
One other thing: The MDPI class file defines that line numbers are added (using the lineno package) when the documentclass option "submit" is chosen. A known limitation of the lineno package is that it doesn't work well with math display (see lineno user's manual at http://www.ctan.org/tex-archive/macros/latex/contrib/lineno/). A common problem, which I also see in the screenshot you posted, is that the lines before the equation are not numbered. The easiest way to circumvent this is to insert a blank line before the equation. FYI: Once the manuscript is accepted for publication, the documentclass option "submit" will be changed to "accept". Then, line numbers are no longer displayed, and the blank line before the equation can be deleted.
Best Answer
I founded the solution.
Simply use the command
\citenum{Key}then I got